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International Journal of Business Marketing and Management (IJBMM)
Volume 8 Issue 2 Mar-Apr 2023, P.P. 91-101
ISSN: 2456-4559
www.ijbmm.com
International Journal of Business Marketing and Management (IJBMM) Page 91
Robust Optimal Reinsurance and Investment Problem
with p-Thinning Dependent and Default risks
Yingqi Liang1
, Peibiao Zhao1
1
School of Mathematics and Statistics,Nanjing University of Science and Technology, Nanjing Jiangsu
210094,China
843186823@
Abstract: In this paper, we consider an AAI with two types of insurance business with p-thinning dependent
claims risk, diversify claims risk by purchasing proportional reinsurance, and invest in a stock with Heston
model price process, a risk-free bond, and a credit bond in the financial market with the objective of maximizing
the expectation of the terminal wealth index effect, and construct the wealth process of AAI as well as the the
model of robust optimal reinsurance-investment problem is obtained, using dynamic programming, the HJB
equation to obtain the pre-default and post-default reinsurance-investment strategies and the display expression
of the value function, respectively, and the sensitivity of the model parameters is analyzed through numerical
experiments to obtain a realistic economic interpretation. The model as well as the results in this paper are a
generalization and extension of the results of existing studies.
Key words: p-Thinning dependent risk, defaultable bonds, dynamic programming, HJB equation, robust
optimization.
I. Introduction
The sound operation and solid claiming ability of insurance companies are important to maintain social
stability and promote social development, and the purchase of reinsurance can well control the risk of insurance
companies. The current research on reinsurance in the actuarial field of insurance mainly focuses on the insurer's
objective function, investment type, claim process, and price process of assets. [1]-[3]studied the most available
reinsurance investment problems under different utility function. [4]-[7]investigate the optimal reinsurance
investment problem under different risky asset price process.
Most of the current literature considers only single-risk claims, while in reality the risks are usually
correlated with each other. The current literature on claim dependency risk is divided into two main types of
dependency, one is co-impact and the other is p-thinning dependency. P-thinning dependency means that when
one type of claim occurs, there is a certain probability p that another type of claim will occur, for example, a car
accident or fire will not only cause property damage, but also loss of life if the situation is serious. The problem
under diffusion approximation model, jump diffusion risk model is studied in[8][9],[10] studied the type of
dependent risk for common shocks, and [11] considered the risk process under p-thinning dependence. Most of
the current studies consider only two assets for considering investments in credit bonds. [12]-[14] take into
account defaulted bonds among the types of investments for insurers. Since models with diffusion and jump
terms usually introduce uncertainty in the model for insurers, insurers usually want to seek a more robust model.
[15]-[17] study the AAI most reinsurance-investment problem.
Based on this, this paper studies the optimal proportional reinsurance-investment problem of AAI under the
Heston model by considering both sparse dependence and claims risk based on [11][14][17]. The paper is
organized as follows: a robust optimal reinsurance-investment problem model under p-thinning dependence and
default risk is developed in Section 2. The value function is divided into pre-default and post-default in Section
3, and the optimal reinsurance-investment strategies are solved for pre-default and post-default using dynamic
programming, optimal control theory, and the HJB equation, respectively. Parameter sensitivities are analyzed,
and economic explanations are given in Section 4. Section 5 concludes the paper.
II. Model Formulation
Suppose *Ξ©, F, *𝐹𝑑+t∈,0,T -, P+ is a complete probability space, the positive number T denotes the final
value moment, [0, T ] is a fixed time interval, 𝐹𝑑 denotes the information in the market up to time t, and 𝔽 ∢=
*𝐹𝑑+t∈,0,T -denotes the standard Brownian motion π‘Š0(𝑑), π‘Š1(𝑑), π‘Š2(𝑑), π‘Š3(𝑑). Poisson process N(t), and the
right-continuous P-complete information flow generated by the sequence of random variables *𝑋𝑖, 𝑖 β‰₯ 1+,
*π‘Œπ‘–, 𝑖 β‰₯ 1+. ℍ ∢= (ℋ𝑑)tβ©Ύ0 is the information flow generated by the violation process H(t), let 𝔾: = (𝒒𝑑)tβ©Ύ0, be
the information flow generated by 𝔽, ℍ the expanded information flow, i.e., 𝔾: = ℱ𝑑 ∨ ℋ𝑑. By definition, each
𝔽 -harness is also the 𝔾 -harness. The probability measure P is a realistic probability measure and Q is a risk-
neutral measure. In addition, it is assumed that all transactions in the financial market are continuous, and no
Robust Optimal Reinsurance and Investment Problem with p-Thinning Dependent and Default ris
International Journal of Business Marketing and Management (IJBMM) Page 92
taxes do not incur transaction costs and all property is infinitely divisible.
1.1 Surplus Process
Assuming that the insurer operates two different lines of business and that there is a sparse dependency between
these two lines of business, the surplus process is as follows:
R(t) = π‘₯0 + 𝑐𝑑 βˆ’ (βˆ‘ 𝑋𝑖
𝑁(𝑑)
𝑖=1
+ βˆ‘ π‘Œπ‘–
𝑁𝑝(𝑑)
𝑖=1
) . #(1)
where{π‘‹π‘–οΌŒπ‘– β‰₯ 1} is independently and identically distributed in 𝐹𝑋(βˆ™),𝐸(𝑋) = πœ‡π‘‹ > 0,𝐸(𝑋2
) = πœπ‘‹
2
, as the
claim amount of the first class of business,{π‘Œπ‘–οΌŒπ‘– β‰₯ 1} is independently and identically distributed inπΉπ‘Œ(βˆ™
),𝐸(π‘Œ) = πœ‡π‘Œ > 0,𝐸(π‘Œ2
) = πœπ‘Œ
2
, as the claim amount of the second class of business. the claim amount of the
second type of business. N(t)denotes the conforming Poisson process with parameter c denotes the premium of
the insurance company by the expected value premium there are c = (1 + ΞΈ1)Ξ»πœ‡π‘‹ + (1 + ΞΈ2)Ξ»pπœ‡π‘Œ.ΞΈ1 > 0,ΞΈ2 >
0.
1.2 Proportional Reinsurance
Assuming that the insurer diversifies the claim risk by purchasing proportional reinsurance, and let π‘ž1(𝑑),π‘ž2(𝑑)
be the insurer's retention ratio, the claim after the insurer purchases reinsurance is:
π‘ž1(𝑑)𝑋𝑖 , π‘ž2(𝑑)π‘Œπ‘– then the reinsurance fee is Ξ΄(π‘ž1(𝑑), π‘ž2(𝑑)) = (1 + Ξ·1)(1 βˆ’ π‘ž1(𝑑))Ξ»πœ‡π‘‹ + (1 + Ξ·2)(1 βˆ’
π‘ž2(𝑑))Ξ»pπœ‡π‘Œ.According to Grandell(1991)[8]
,the claims process can be diffusely approximated as:
d βˆ‘ Xi
N(t)
i=1
= Ξ»E(Xi)dt βˆ’ Ξ³1dWX(t), Ξ³1 = √λE(XI
2
)
d βˆ‘ Yi
Np(t)
i=1
= Ξ»E(Yi)dt βˆ’ Ξ³2dWY(t), Ξ³2 = √λE(YI
2
)
The correlation coefficient of π‘Šπ‘‹(𝑑)π‘Šπ‘Œ(𝑑 )is ρ
Μ‚ =
Ξ»p
𝛾1𝛾2
E(𝑋𝑖)E(π‘Œπ‘–).Then the wealth process of the insurer after
joining the reinsurance is:
dXq1,q2 = ,λμX(ΞΈ1 βˆ’ Ξ·1 + Ξ·1π‘ž1(𝑑)) + Ξ»pΞΌY(ΞΈ2 βˆ’ Ξ·2
+Ξ·2π‘ž2(𝑑))𝑑𝑑 + √(π‘ž1𝛾1)2 + (π‘ž2𝛾2)2 + 2ρ
Μ‚π‘ž1π‘ž2𝛾1𝛾2dW0
1.3 Financial Market
Suppose the financial market consists of three assets: risk-free assets, stocks, and corporate bonds, and the price
processes of the three assets are as follows: The price process of risk-free bonds is given byοΌšπ‘‘R(t) =
rR(t)dt.The stock price process S(t) obeys the Heston stochastic volatility model:
{
𝑑𝑆(𝑑) = 𝑆(𝑑),π‘Ÿ + 𝛼𝐿(𝑑)𝑑𝑑 + √𝐿(𝑑)π‘‘π‘Š1(𝑑)-, 𝑆(0) = 𝑠0
𝑑𝐿(𝑑) = π‘˜(πœ” βˆ’ 𝐿(𝑑))𝑑𝑑 + 𝜍√𝐿(𝑑)π‘‘π‘Š2(𝑑), 𝐿(0) = 𝑙0
R is the risk-free rate,Ξ±, π‘˜, Ο‚, are positive constant.𝐸,π‘Š1π‘Š2- = ρt, 2π‘˜πœ” β‰₯ Ο‚2
Following Bielecki (2007)[18]
, the credit bond price process p(t, T1), under a realistic measure P, using an
approximate model to portray default risk is as follows:
dp(t, T1) = p(tβˆ’, T1),(r + (1 βˆ’ H(t))Ξ΄(1 βˆ’ Ξ”)dt
βˆ’(1 βˆ’ H(tβˆ’))ΞΆdMp
(t)-
Where Mp
(t) = H(t) βˆ’ hQ
∫ Ξ”(1 βˆ’ H(u))du
t
0
is aβ„Š βˆ’ harness, Ξ΄ = 𝑕𝑄
ΞΆ is the credit spread.
1.4 Robust optimization problem
Assuming the insurer adopts a reinsurance investment strategyΞ΅(t) = (π‘ž1(𝑑), π‘ž2(𝑑), Ο€(t), Ξ²(𝑑))οΌŒπ‘ž1(𝑑), π‘ž2(𝑑)are
the reinsurance strategies adopted by the insurer at moment t in the first and second asset classes,
respectively.Ο€(t) for the insurance company's investment in equities at time t.,β(𝑑)for the insurance companyβ€²s
investment in credit bonds at time t. Let 𝔸 denotes the set of all feasible strategies, then the dynamic process of
wealth of the insurance company at this moment π‘‹πœ€
(t) is:
dπ‘‹πœ€(𝑑) = πœ‹(𝑑)
𝑑𝑆(𝑑)
𝑆(𝑑)
+ Ξ²(𝑑)
𝑑𝑝(𝑑)
𝑝(𝑑)
+ (π‘‹πœ€(𝑑) βˆ’ πœ‹(𝑑) βˆ’ Ξ²(𝑑))
𝑑𝐡(𝑑)
𝐡(𝑑)
Robust Optimal Reinsurance and Investment Problem with p-Thinning Dependent and Default ris
International Journal of Business Marketing and Management (IJBMM) Page 93
+π‘‘π‘‹π‘š(𝑑),XΞ΅(0) = x0
Assume that the insurer maximizes the terminal T moment expected utility in the financial market, the portfolio
index utility, which takes the form of:
V(X(T)) = βˆ’
1
𝛾
π‘’βˆ’π›Ύπ‘‹(𝑇)
Where 𝛾 > 0 is the ambiguity aversion coefficient of the insurer. The insurer's goal is to find the optimal
reinsurance-investment strategyπœ€βˆ—
(𝑑) = (π‘ž1
βˆ—
(𝑑), π‘ž2
βˆ—
(𝑑), πœ‹βˆ—
(𝑑), Ξ²βˆ—
(𝑑))to maximizing the expectation of end-use
wealth utility for insurers. The objective function of the insurer is:
VΞ΅
(t, x, l, h) = E(u(XΞ΅
(T))|XΞ΅
(t) = x, L(t) = l)
The value function of the optimization problem is:
V(t, x, l, h) = VΞ΅
(t, x, l, h)
Ρ∈Π
sup
, V(T,x,l,h)=v(x),
Assume that the ambiguity information is described by the probability P and the reference model is measured by
the probability 𝒫Φ
which is equivalent to PοΌšπ’«: = *𝒫Φ
|𝒫Φ
~𝑃+.Next, the optional measure set is constructed,
defining the procedure :Ξ¦(𝑑) = (Ξ¦0(𝑑), Ξ¦1(𝑑), Ξ¦2(𝑑), Ξ¦3(𝑑))s.t.:
1. Ξ¦(𝑑) is a β„Š(𝑑)- measurable for any t[0,T]
2. Ξ¦i(t) = Ξ¦i(t, Ο‰), i = 0,1,2,3 andΞ¦i(t) β‰₯ 0 for all (t, Ο‰) ∈ ,0, T- Γ— Ξ©.
3.∫ ||Φ(t)||2
𝑑𝑑 < ∞;
𝑇
0
Let Σ be all processes shaped as Φ,for all Φ ∈ Σ define a G-adaptation process under a real measure
*ΛΦ
(𝑑)|𝑑 ∈ ,0, T-+.From Ito differentiation we have:
𝑑ΛΦ
(𝑑) = ΛΦ
(π‘‘βˆ’)(βˆ’Ξ¦0(𝑑)dW0 βˆ’ Ξ¦1(𝑑)π‘‘π‘Š1 βˆ’ Ξ¦2(𝑑)π‘‘π‘Š2 βˆ’ (1 βˆ’ Ξ¦3(𝑑))𝑑Mp
)
Where ΛΦ
(0) = 1,P-a.s. ΛΦ
(𝑑) is a (P,G)- martingale,E[ΛΦ
(𝑇)] =1, for each Ξ¦ ∈ Ξ£,A new optional measure
is absolutely continuous to P, defined as
𝑑𝑃Φ
𝑑𝑃
|𝒒𝑇
= ΛΦ
(𝑇).
So far, we have constructed a class of real-world probability measures 𝑃Φ
,where Φ ∈ Σ,From Gisanova's
theorem[21]
,it follows that:
𝑑Wi
𝑃Φ
(𝑑) = dWi(𝑑) + Φ𝑖(𝑑)𝑑𝑑, 𝑖 = 0,1,2
Therefore the wealth process in the 𝑃Φ
is:
dπ‘‹πœ€
(𝑑) = ,xr + πœ‹(𝛼𝑙 βˆ’ Ξ¦1βˆšπ‘™) + Ξ²(1 βˆ’ H(t))Ξ΄(1 βˆ’ Ξ”)
+λμX(ΞΈ1 βˆ’ Ξ·1) + Ξ»pΞΌY((ΞΈ2 βˆ’ Ξ·2) + Ξ·1π‘ž1(𝑑)) + Ξ·2π‘ž2(𝑑))
+√(π‘ž1𝛾1)2 + (π‘ž2𝛾2)2 + 2ρ
Μ‚π‘ž1π‘ž2𝛾1𝛾2Ξ¦0-dt + πœ‹βˆšπ‘™π‘‘W1
𝑃Φ
+Ξ²(1 βˆ’ H(tβˆ’))ΞΆdMp
+ √(π‘ž1𝛾1)2 + (π‘ž2𝛾2)2 + 2ρ
Μ‚π‘ž1π‘ž2𝛾1𝛾2𝑑W0
𝑃Φ
(2)
We assume that the insurer determines a robust portfolio strategy such that the worst-case scenario is the best
option. The insurer penalizes any deviation from the sub-reference model with a penalty that increases with this
deviation, using relative entropy to measure the deviation between the reference measure and the optional
measure. Inspired by Maenhout[19]
Branger[20]
the problem is modified to define the value function as:
V(t, x, l. h) = 𝐸𝑑,π‘₯,𝑙,β„Ž
𝑃Φ
0βˆ’
1
𝛾
π‘’βˆ’π›Ύπ‘‹(𝑇)
+ ∫ 𝐺(𝑒, π‘‹πœ€(𝑒), πœ™(𝑒))𝑑𝑒
𝑇
𝑑
1
Φ∈Σ
𝑖𝑛𝑓
πœ€βˆˆπ›±
𝑠𝑒𝑝
𝐸𝑑,π‘₯,𝑙,β„Ž
𝑃Φ
Calculated under the optional measure , the initial value of the stochastic process is π‘‹πœ€(𝑑)=x, S(t)=s,
H(t)=h,
G(u, XΞ΅(u), Ο•(u)) =
Ξ¦0
2
2Ξ¨0(t, XΞ΅(t), Ο•(t))
+
Ξ¦1
2
2Ξ¨1(t, XΞ΅(t), Ο•(t))
+
Ξ¦2
2
2Ξ¨2(t, XΞ΅(t), Ο•(t))
+
(Ξ¦3 ln Ξ¦3 βˆ’ Ξ¦3 + 1)hp
(1 βˆ’ h)
2Ξ¨3(t, XΞ΅(t), Ο•(t))
Where Ξ¨0 β‰₯ 0,Ψ1 β‰₯ 0,Ψ2 β‰₯ 0,Ψ3 β‰₯ 0 is state-dependent,let Ξ¨0 = βˆ’
𝑣0
𝛾V(t,x,l,h)
, Ξ¨1 = βˆ’
𝑣1
𝛾V(t,x,l,h)
, Ξ¨2 =
βˆ’
𝑣2
𝛾V(t,x,l,h)
, Ξ¨3 = βˆ’
𝑣3
𝛾V(t,x,l,h)
𝑣𝑖,i=0,1,2,3 is the risk aversion factor, the lager Ξ¨i. According to the dynamic planning principle, the HJB
equation is as follows:
π’œπœ€,πœ™
𝑉 + 𝐺(𝑒, π‘‹πœ€(𝑒), πœ™(𝑒)) = 0
Ξ¦πœ–Ξ£
𝑖𝑛𝑓
πœ€πœ–βˆ
𝑠𝑒𝑝
#(3)
π’œπœ€,πœ™
is the infinitesimal operator under the measure 𝑃Φ
.
Robust Optimal Reinsurance and Investment Problem with p-Thinning Dependent and Default ris
International Journal of Business Marketing and Management (IJBMM) Page 94
III. Robust optimal reinsurance-investment strategy solving
This section will solve the robust optimal problem constructed in the previous section. This paper
divides the value function into pre-default and post-default components according to the time of default of the
credit bond:
V(T, x, l, h) = {
V(T, x, l, 0),h = 0(before default)
V(T, x, l, 1),h = 1(after default)
By decomposing the value function into two sub-functions, denoted as the value function before the zero-
coupon bond default and the value function after the zero-coupon bond default, the two sub-HJB equations are
obtained and solved successively to obtain the reinsurance and risky asset investment strategies and value
function expressions after default, and the reinsurance, risky asset and credit bond investment strategies and
value function expressions before default.
1.5 Optimal reinsurance and investment decisions after default
When H (t)=1οΌŒΟ„ ∧ T ≀ t ≀ T,the insurer has constituted a default at or before time t, the HJB equation
degenerates to:
𝑉𝑑 +
[π‘Ÿπ‘₯ + πœ‹(𝛼𝑙 βˆ’ Ξ¦1βˆšπ‘™) + πœ†ΞΌX(ΞΈ1 βˆ’ Ξ·1) + Ξ»pΞΌY(ΞΈ2 βˆ’ Ξ·2) + πœ†ΞΌXΞ·1π‘ž1(𝑑) + Ξ»pΞΌYΞ·2π‘ž2(𝑑) +
√(π‘ž1𝛾1)2 + (π‘ž2𝛾2)2 + 2ρ
Μ‚π‘ž1π‘ž2𝛾1𝛾2Ξ¦0]𝑉
π‘₯ +
1
2
(Ο€2
l + Ξ»(q1Ο‚X)2
+ Ξ»p(q2Ο‚Y)2
+ 2Ξ»pΞΌXΞΌY)Vxx + Ο€lςρVxl +
(k(Ο‰ βˆ’ l) βˆ’ Ξ¦1πœπœŒβˆšπ‘™ βˆ’ Ξ¦2𝜍√1 βˆ’ 𝜌2βˆšπ‘™)Vl +
1
2
Ο‚2
lVll βˆ’
Ξ¦0
2
2v0
𝛾 βˆ’
Ξ¦1
2
2v1
𝛾 βˆ’
Ξ¦2
2
2v2
𝛾 = 0 (4)
Satisfied :V(T, x, l, 1) = βˆ’
1
Ξ³
eβˆ’Ξ³X
The solution can be assumed to be of the form:
V(t, x, y, l, 1) = βˆ’
1
Ξ³
exp*βˆ’Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
x + G(t, l)+, G(T, l) = 0#(5)
Taking each partial derivative of V:
{
𝑉𝑑 = [Ξ³rxπ‘’π‘Ÿ(π‘‡βˆ’π‘‘)
+ Gt]V, 𝑉
π‘₯ = βˆ’Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
V
𝑉
π‘₯π‘₯ = Ξ³2
𝑒2π‘Ÿ(π‘‡βˆ’π‘‘)
π‘‰οΌŒπ‘‰π‘₯𝑙 = βˆ’Ξ³πΊπ‘™π‘‰
𝑉𝑙 = 𝐺𝑙𝑉, 𝑉𝑙𝑙 = 𝐺𝑙𝑙𝑉 + 𝐺𝑙
2
𝑉
The minimum value point of Ξ¦βˆ—
is obtained from the first order condition as:
{
Ξ¦0
βˆ—
= v0er(Tβˆ’t)
√(q1γ1)2 + (q2γ2)2 + 2ρ
Μ‚q1q2Ξ³1Ξ³2
Ξ¦1
βˆ—
= v1er(Tβˆ’t)
Ο€βˆšl, Ξ¦2
βˆ—
= βˆ’Ο‚ΟβˆšlGlv2
1
Ξ³
Bringing in the HJB equation yields:
Ξ³rxπ‘’π‘Ÿ(π‘‡βˆ’π‘‘)
+ Gt βˆ’ ,λμX(ΞΈ1 βˆ’ Ξ·1) + Ξ»pΞΌY(ΞΈ2 βˆ’ Ξ·2)-
Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
+ k(Ο‰ βˆ’ l)𝐺𝑙 +
(πœπœŒβˆšπ‘™πΊπ‘™)
2
v2
2
+
1
2
𝜍2
𝑙(𝐺𝑙𝑙 + 𝐺𝑙
2
)
+ *𝑓2(πœ‹, 𝑑)+
Ο€
inf
+ {Ξ»Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
𝑓1(π‘ž1, π‘ž2, 𝑑)}
π‘ž1,π‘ž2
inf
= 0#(6)
Where
𝑓1(π‘ž1, π‘ž2, 𝑑) = βˆ’,ΞΌXΞ·1π‘ž1(𝑑) + pΞΌYΞ·2π‘ž2(𝑑)-
+
(Ξ³ + v0)π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
2
,(π‘ž1πœπ‘‹)2
+ p(π‘ž2πœπ‘Œ)2
+ 2pΟ‚XΟ‚Yπ‘ž1π‘ž2-
𝑓2(πœ‹, 𝑑) = πœ‹π‘™π‘’π‘Ÿ(π‘‡βˆ’π‘‘)(ςρ𝐺𝑙(Ξ³ + v1) βˆ’ 𝛼) +
1
2
(Ξ³ + v1)lΟ€2
𝑒2π‘Ÿ(π‘‡βˆ’π‘‘)
Theorem III.1 Let m=
ΞΌX(Ξ·1πœŽπ‘Œ
2βˆ’pΞ·2ΞΌY
2)
𝜎π‘₯
2πœŽπ‘Œ
2βˆ’π‘ΞΌX
2ΞΌY
2 , n=
ΞΌY(Ξ·2πœŽπ‘‹
2βˆ’Ξ·1ΞΌX
2)
𝜎π‘₯
2πœŽπ‘Œ
2βˆ’π‘ΞΌX
2ΞΌY
2 ,then there exist 𝑑1, 𝑑2𝑑1
Μ‚, 𝑑2
Μ‚ and the following values
can be obtained:
Robust Optimal Reinsurance and Investment Problem with p-Thinning Dependent and Default ris
International Journal of Business Marketing and Management (IJBMM) Page 95
𝑑1 = 𝑇 βˆ’
1
π‘Ÿ
ln
π‘š
Ξ³ + v0
, 𝑑2 = 𝑇 βˆ’
1
π‘Ÿ
ln
𝑛
Ξ³ + v0
𝑑1
Μ‚ = 𝑇 βˆ’
1
π‘Ÿ
ln
ΞΌXΞ·1
(Ξ³ + v0)p(πœπ‘‹
2 + ΞΌXΞΌY)
𝑑2
Μ‚ = 𝑇 βˆ’
1
π‘Ÿ
ln
ΞΌyΞ·2
(Ξ³ + v0)(πœπ‘Œ
2 + ΞΌXΞΌY)
When m ≀ Ξ³(n ≀ Ξ³),let 𝑑2 = 𝑇(𝑑1 = 𝑇).When m > Ξ³(n > Ξ³),let 𝑑2 = 0(𝑑1 = 0)(1)If π‘š ≀ n,then π‘ž1
βˆ—
≀ π‘ž2
βˆ—
,for
all tΟ΅,0, T-,The reinsurance strategy corresponding to the problem is(π‘ž1
βˆ—
, π‘ž2
βˆ—
) = {
(π‘ž1
Μ‚ , π‘ž2
Μ‚),0 ≀ 𝑑 ≀ 𝑑2
(π‘ž1
Μƒ, 1
Μƒ)οΌŒπ‘‘2 ≀ 𝑑 ≀ 𝑑1
Μ‚
(1,1)οΌŒπ‘‘1 ≀ 𝑑 ≀ 𝑇
(2)If
n > m,then for alltϡ,0, T-,The reinsurance strategy corresponding to the problem
is(π‘ž1
βˆ—
, π‘ž2
βˆ—
) = {
(π‘ž1
Μ‚ , π‘ž2
Μ‚),0 ≀ 𝑑 ≀ 𝑑1
(1, π‘ž2
Μƒ), 𝑑1 ≀ 𝑑 ≀ 𝑑2
Μ‚
(1,1), 𝑑2
Μ‚ ≀ 𝑑 ≀ 𝑇
. Where
π‘ž1
Μ‚ =
ΞΌX(Ξ·1πœπ‘Œ
2
βˆ’ pΞ·2ΞΌY
2)
(Ξ³ + v0)π‘’π‘Ÿ(π‘‡βˆ’π‘‘)(𝜍π‘₯
2πœπ‘Œ
2 βˆ’ 𝑝μX
2ΞΌY
2)
#(7)
π‘ž2
Μ‚ =
ΞΌY(Ξ·2πœπ‘‹
2
βˆ’ Ξ·1ΞΌX
2)
(Ξ³ + v0)π‘’π‘Ÿ(π‘‡βˆ’π‘‘)(𝜍π‘₯
2πœπ‘Œ
2 βˆ’ 𝑝μX
2ΞΌY
2)
#(8)
π‘ž1
Μƒ =
ΞΌXΞ·1 βˆ’ pΞΌXΞΌY(Ξ³ + v0)π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
πœπ‘‹
2(Ξ³ + v0)π‘’π‘Ÿ(π‘‡βˆ’π‘‘)p
#(9)
π‘ž2
Μƒ =
ΞΌyΞ·2 βˆ’ ΞΌXΞΌY(Ξ³ + v0)π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
πœπ‘Œ
2(Ξ³ + v0)π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
#(10)
Proof:By finding the first-order partial derivatives, second-order partial derivatives and second-order mixed
partial derivatives for𝑓1, the following system of equations and the Hessian array are obtained:
{
πœ•π‘“1
πœ•π‘ž1
= βˆ’ΞΌXΞ·1 + (Ξ³ + v0)π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
,πœπ‘‹
2
π‘ž1 + pΟ‚XΟ‚Yπ‘ž2- = 0
πœ•π‘“1
πœ•π‘ž2
= βˆ’ΞΌYΞ·2 + (Ξ³ + v0)π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
,πœπ‘Œ
2
π‘ž2 + Ο‚XΟ‚Yπ‘ž1- = 0
(11)
|
|
πœ•2
𝑓1
πœ•π‘ž1πœ•π‘ž1
πœ•2
𝑓1
πœ•π‘ž1πœ•π‘ž2
πœ•2
𝑓1
πœ•π‘ž2πœ•π‘ž1
πœ•2
𝑓1
πœ•π‘ž2πœ•π‘ž2
|
| = 𝑒4π‘Ÿ(π‘‡βˆ’π‘‘)
(Ξ³ + v0)2
|
βˆ’πœπ‘₯
2
(Ξ³ + v0) βˆ’pΟ‚XΟ‚Y
βˆ’pΟ‚XΟ‚Y βˆ’π‘πœπ‘¦
2
(Ξ³ + v0)
|
From the Hessian array positive definite it is known that 𝑓1(π‘ž1, π‘ž2, 𝑑) is a convex function and there exist
extreme value points; solving the system of equations (11) yields (7). (8).
ObviouslyοΌŒπ‘ž1
Μ‚ γ€π‘ž2
Μ‚ are both increasing functions with respect to t, when π‘š ≀ n,0 ≀ 𝑑 ≀ 𝑑1 or n > m,0 ≀
𝑑 ≀ 𝑑2the solution as (7)(8),Also the values of 𝑑1 and 𝑑2 can be found. When π‘š ≀ nοΌŒπ‘‘2 ≀ 𝑑 ≀ 𝑑1
Μ‚ ,then
(q1
βˆ—
, q2
βˆ—
) = (π‘ž1
Μƒ, 1),q1 ∈ ,0,1-.When 𝑛 > mοΌŒπ‘‘1 ≀ 𝑑 ≀ 𝑑2
Μ‚ then (q1
βˆ—
, q2
βˆ—
) = (1, π‘ž1
Μƒ), q2 ∈ ,0,1-,Separate solve
df1(t,q1,1)
dq1
= 0,
df1(t,1,q2)
dq2
= 0 , then can get π‘ž1
Μƒ, π‘ž2
Μƒ as (9), (10) End.
Theorem III.2 The post-default insurer's optimal risky asset investment strategy is to
Ο€βˆ—
=
Ξ± βˆ’ ςρ(Ξ³ + v1)𝐺1
(Ξ³ + v1)π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
#(12)
The expression of the optimal value function is:
V(t, x, y, l, 1) = βˆ’
1
Ξ³
exp*βˆ’Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘)(t)x + G1(t)l + G2(t)+#(13)
π‘€π‘•π‘’π‘Ÿπ‘’ ρ β‰  Β±1, 𝐺1 =
𝑙1𝑙2 βˆ’ 𝑙1𝑙2π‘’βˆ’
1
2
Ο‚2(𝑙1βˆ’π‘™2)(Ξ³+v1)(1βˆ’Ο2)(π‘‡βˆ’π‘‘)
𝑙2 βˆ’ 𝑙1π‘’βˆ’
1
2
Ο‚2(𝑙1βˆ’π‘™2)(Ξ³+v1)(1βˆ’Ο2)(π‘‡βˆ’π‘‘)
, #(14)
ρ = 1, 𝐺1 =
Ξ±2
2(Ξ³ + v1)(Ξ±Ο‚ + k)
(1 βˆ’ π‘’βˆ’(Ξ±Ο‚+k)(π‘‡βˆ’π‘‘)
), #(15)
Robust Optimal Reinsurance and Investment Problem with p-Thinning Dependent and Default ris
International Journal of Business Marketing and Management (IJBMM) Page 96
ρ = βˆ’1, k β‰  Ξ±Ο‚, 𝐺1 =
Ξ±2
2(Ξ³ + v1)(π‘˜ βˆ’ Ξ±Ο‚)
(1 βˆ’ 𝑒(Ξ±Ο‚βˆ’k)(π‘‡βˆ’π‘‘)
), #(16)
ρ = βˆ’1, π‘˜ = Ξ±Ο‚, 𝐺1 =
Ξ±2
2(Ξ³ + v1)
, #(17)
𝑙1,2 =
βˆ’(αςρ + k) Β± √(αςρ + k)2 + Ξ±2Ο‚2(1 βˆ’ ρ2)
Ξ³Ο‚2(1 βˆ’ ρ2)
G2 =
Ξ³
r
,Ξ»(ΞΌX(ΞΈ1 βˆ’ Ξ·1) + Ξ»p(ΞΌY(ΞΈ2 βˆ’ Ξ·2)-,1 βˆ’ π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
-
+π‘˜πœ” ∫ G1(s)ds
𝑇
𝑑
+ ∫ Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
𝑓1(π‘ž1
βˆ—
, π‘ž2
βˆ—
, 𝑠)𝑑𝑠
𝑇
𝑑
#(18)
Proof: Let
G(t, l) = G1(t)l + G2(t)#(19)
Bringing equation (19) into equation (6) can be obtained as the following two equations:
G1
’
βˆ’
Ξ±2
2(Ξ³ + v1)
βˆ’ G1(αςρ + k) +
1
2
(Ξ³ + v1)Ο‚2
G1
2
(ρ2
βˆ’ 1) = 0#(20)
G2β€˜ βˆ’ ,Ξ»(ΞΌX(ΞΈ1 βˆ’ Ξ·1) + Ξ»p(ΞΌY(ΞΈ2 βˆ’ Ξ·2)-Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
+π‘˜πœ”G1 + Ξ»Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
𝑓1(π‘ž1
βˆ—
, π‘ž2
βˆ—
, 𝑑) = 0#(21)
Solving equation (20) yields equations (14)-(17). From equation (21) we get (18), and the specific expression of
(18) is discussed in the following cases:
1)When π‘š ≀ n,0 ≀ 𝑑 ≀ 𝑑2(n > m, 0 ≀ 𝑑 ≀ 𝑑1)
G2 =
Ξ³
π‘Ÿ
,Ξ»(ΞΌX(ΞΈ1 βˆ’ Ξ·1) + Ξ»p(ΞΌY(ΞΈ2 βˆ’ Ξ·2)-,π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
βˆ’ 1- + G1(t)
Μ‚
βˆ’Ξ»(T βˆ’ t)*ΞΌXΞ·1
ΞΌX(Ξ·1πœπ‘Œ
2
βˆ’ pΞ·2ΞΌY
2)
(𝜍π‘₯
2πœπ‘Œ
2 βˆ’ 𝑝μX
2ΞΌY
2)
+ pΞΌYΞ·2
ΞΌY(Ξ·2πœπ‘‹
2
βˆ’ pΞ·1ΞΌX
2)
(𝜍π‘₯
2πœπ‘Œ
2 βˆ’ 𝑝μX
2ΞΌY
2)
βˆ’
1
2
,(
ΞΌXπœπ‘‹(Ξ·1πœπ‘Œ
2
βˆ’ pΞ·2ΞΌY
2)
(𝜍π‘₯
2πœπ‘Œ
2 βˆ’ 𝑝μX
2ΞΌY
2)
)
2
+ p (
ΞΌYπœπ‘Œ(Ξ·2πœπ‘‹
2
βˆ’ pΞ·1ΞΌX
2)
(𝜍π‘₯
2πœπ‘Œ
2 βˆ’ 𝑝μX
2ΞΌY
2)
)
2
+2pΟ‚XΟ‚Y
ΞΌX(Ξ·1πœπ‘Œ
2
βˆ’ pΞ·2ΞΌY
2
)
(𝜍π‘₯
2πœπ‘Œ
2 βˆ’ 𝑝μX
2ΞΌY
2)
ΞΌY(Ξ·2πœπ‘‹
2
βˆ’ pΞ·1ΞΌX
2
)
(𝜍π‘₯
2πœπ‘Œ
2 βˆ’ 𝑝μX
2ΞΌY
2)
-+
2οΌ‰When π‘š ≀ 𝑛, 𝑑2 ≀ 𝑑 ≀ 𝑑1
Μ‚,
G2 =
Ξ³
r
,Ξ»(ΞΌX(ΞΈ1 βˆ’ Ξ·1) + Ξ»p(ΞΌY(ΞΈ2 βˆ’ Ξ·2)-,π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
βˆ’ 1-
+
πœ†π›Ύ
(𝛾 + v0)
(
ΞΌXΞ·1
πœπ‘‹π‘
)
2
(
1
2
βˆ’ p)(T βˆ’ t) + Ξ»(βˆ’
ΞΌXΞ·1pΞΌXΞΌY
πœπ‘‹
2p
βˆ’Ξ»pΞΌYΞ·2 +
ΞΌXΞ·1ΞΌXΞΌY
πœπ‘‹
+ Ο‚Y
ΞΌXΞ·1
Ο‚X
)
Ξ³
r
(π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
βˆ’ 1)
+,(
ΞΌXΞΌY
πœπ‘‹
)
2
+ pπœπ‘Œ
2
βˆ’ 2𝑝ςY
ΞΌXΞΌY
πœπ‘‹
-
λγ(𝛾 + v0)
2r
(𝑒2π‘Ÿ(π‘‡βˆ’π‘‘)
βˆ’ 1) + π‘˜πœ”G1(t)
Μ‚
3οΌ‰When π‘š > 𝑛, 𝑑1 ≀ 𝑑 ≀ 𝑑2
Μ‚,optimal reinsurance strategy (π‘ž1
βˆ—
, π‘ž2
βˆ—
) =(1,π‘ž2
Μƒ),
G2 =
Ξ³
r
,Ξ»(ΞΌX(ΞΈ1 βˆ’ Ξ·1) + Ξ»p(ΞΌY(ΞΈ2 βˆ’ Ξ·2)-,π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
βˆ’ 1-
+
λγ
(γ + 𝜈0)
(
ΞΌyΞ·2
πœπ‘Œ
)
2
(
1
2p
βˆ’ 1) (T βˆ’ t) + π‘˜πœ”G1(t)
Μ‚
+λμyΞ·2 (ΞΌXΞ·1 βˆ’ 𝑝μYΞ·2
ΞΌXΞΌY
πœπ‘Œ
2
βˆ’ Ο‚X
ΞΌyΞ·2
πœπ‘Œ
+
ΞΌyΞ·2ΞΌXΞΌY
2πœπ‘Œ
2
)
Ξ³(π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
βˆ’ 1)
r
+(πœπ‘‹
2
βˆ’ 2pΟ‚X
ΞΌXΞΌY
πœπ‘Œ
+ p(
ΞΌXΞΌY
πœπ‘Œ
)2
)
λ𝛾(𝛾 + v0)
4r
(𝑒2π‘Ÿ(π‘‡βˆ’π‘‘)
βˆ’ 1)
4) When π‘š > 𝑛, 𝑑2
Μ‚ ≀ 𝑑 ≀ T(π‘š ≀ 𝑛, 𝑑1
Μ‚ ≀ 𝑑 ≀ 𝑇), optimal reinsurance strategy (π‘ž1
βˆ—
, π‘ž2
βˆ—
) =(1,1),
G2 =
Ξ³
r
,λμXΞΈ1 + Ξ»pΞΌYΞΈ2-,π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
βˆ’ 1-
+π‘˜πœ”G1(t)
Μ‚ +
λγ(γ + v0)
4
,(πœπ‘‹)2
+ p(πœπ‘Œ)2
+ 2pΟ‚XΟ‚Y-(𝑒2π‘Ÿ(π‘‡βˆ’π‘‘)
βˆ’ 1)
Where G1(t)
Μ‚ = kΟ‰ ∫ G1(s)ds
T
t
=
𝑙2π‘˜πœ”(𝑇 βˆ’ 𝑑) βˆ’ 2π‘˜πœ”(𝜍2
(1 βˆ’ 𝜌2
)βˆ’1
Robust Optimal Reinsurance and Investment Problem with p-Thinning Dependent and Default ris
International Journal of Business Marketing and Management (IJBMM) Page 97
βˆ— 𝑙𝑛|
𝑙1 βˆ’ 𝑙2
𝑙1 βˆ’ 𝑙2𝑒0.5𝜎2(𝑙1βˆ’π‘™2)(1βˆ’πœŒ2)(Ξ³+v1)(π‘‡βˆ’π‘‘)
|),ρ β‰  Β±1
Ξ±2
π‘˜πœ”
2(π‘˜ + Ξ±Ο‚)(Ξ³ + v1)
(T βˆ’ t) +
1
2
(
Ξ±
(k + Ξ±Ο‚)(Ξ³ + v1)
)2
π‘˜πœ”(1 βˆ’ 𝑒(π‘˜+Ξ±Ο‚)(π‘‡βˆ’π‘‘)
),ρ = 1
Ξ±2
π‘˜πœ”
2(π‘˜ βˆ’ Ξ±Ο‚)(Ξ³ + v1)
(T βˆ’ t) +
1
2
(
Ξ±
(k βˆ’ Ξ±Ο‚)(Ξ³ + v1)
)2
π‘˜πœ”(1 βˆ’ 𝑒(π‘˜βˆ’Ξ±Ο‚)(π‘‡βˆ’π‘‘)
),ρ = βˆ’1, k β‰  Ξ±Ο‚
π‘˜πœ”
,(Tβˆ’t)Ξ±-2
4(Ξ³+v1)
,ρ = βˆ’1,k = Ξ±Ο‚. End.
1.6 Optimal reinsurance and investment decisions before default
This section considers the optimal pre-default reinsurance-investment strategy and the value function expression
based on the previous section, when H(t)=0, 0 ≀ t ≀ Ο„ ∧ T.Let the solution of the default prior value function
have the following form:
V(T, x, l, 0) = βˆ’
1
𝛾
𝑒π‘₯𝑝{βˆ’π›Ύπ‘’π‘Ÿ(π‘‡βˆ’π‘‘)(𝑑)π‘₯ + 𝐾(𝑑, 𝑙)}#(22)
Satisfying the boundary conditions V(T, x, l, 0)=V(x), K(T, l)=0, the HJB equation is transformed into:
𝑉𝑑 + ,πœ‹(𝛼𝑙 βˆ’ Ξ¦1βˆšπ‘™) + Ξ²Ξ΄(1 βˆ’ Ξ”) + λμX(ΞΈ1 + Ξ·1π‘ž1(𝑑) βˆ’ 1))
+Ξ»pΞΌY .ΞΈ2 + Ξ·2(π‘ž2(𝑑) βˆ’ 1)√(π‘ž1𝛾1)2 + (π‘ž2𝛾2)2 + 2ρ
Μ‚π‘ž1π‘ž2𝛾1𝛾2Ξ¦01 𝑉
π‘₯
+
1
2
((πœ‹)2
𝑙 + Ξ»(π‘ž1πœπ‘‹)2
+ Ξ»p(π‘ž2πœπ‘Œ)2
+ 2Ξ»pπœπ‘‹πœπ‘Œ)𝑉
π‘₯π‘₯
+πœ‹π‘™πœΟπ‘‰π‘₯𝑙 + (k(Ο‰ βˆ’ l) βˆ’ Ξ¦1πœπœŒβˆšπ‘™ βˆ’ Ξ¦2𝜍√1 βˆ’ 𝜌2βˆšπ‘™))Vl +
1
2
Ο‚2
lVll
+𝑉(𝑒𝛾β(t)΢𝐻1(𝑑)+𝐾(𝑑,𝑙)βˆ’G(𝑑,𝑙)
βˆ’ 1)𝑕𝑝
βˆ’
Ξ¦0
2
2v0
𝛾 βˆ’
Ξ¦1
2
2v1
𝛾 βˆ’
Ξ¦2
2
2v2
𝛾 βˆ’
(Ξ¦3 ln Ξ¦3 βˆ’ Ξ¦3 + 1)hp
(1 βˆ’ h)
2v2
𝛾 = 0
Similarly find the partial derivative of V:
{
𝑉𝑑 = [Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
+ Kt’]V, 𝑉
π‘₯ = βˆ’Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
V
𝑉
π‘₯π‘₯ = Ξ³2
𝑒2π‘Ÿ(π‘‡βˆ’π‘‘)
π‘‰οΌŒπ‘‰π‘₯𝑙 = βˆ’Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
𝐾𝑙𝑉
𝑉𝑙 = 𝐾𝑙𝑉, 𝑉𝑙𝑙 = 𝐾𝑙𝑙𝑉 + 𝐾𝑙
2
𝑉
Bringing the above expression into the HJB equation and fixing the reinsurance-investment strategy, the
minimum point of Ξ¦ is obtained according to the first-order condition as:
{
Ξ¦0
βˆ—
= v0π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
√(π‘ž1𝛾1)2 + (π‘ž2𝛾2)2 + 2ρ
Μ‚π‘ž1π‘ž2𝛾1𝛾2
Ξ¦1
βˆ—
= v1π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
πœ‹βˆšπ‘™, Ξ¦2
βˆ—
= βˆ’πœπœŒβˆšπ‘™K𝑙v2
1
𝛾
Ξ¦3
βˆ—
= 𝑒π‘₯𝑝*
v3
𝛾
.π‘’βˆ’π›ΎΞ²(t)ΞΆπ‘’π‘Ÿ(π‘‡βˆ’π‘‘)+𝐺1(𝑑,𝑙)βˆ’G(𝑑,𝑙)
βˆ’ 1/+
#(23)
After bringing (23) into the HJB equation, by inverting the investment strategy we can obtain:
Ο€βˆ—
=
Ξ± βˆ’ ςρ(Ξ³ + v1)Kl
(Ξ³ + v1)π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
, Ξ²βˆ—
=
ln
1
βˆ†Ξ¦3
+ K(t, l) βˆ’ G(t, l)
ΞΆΞ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
and obviously the expressions for the reinsurance strategy before and after the bond default are the same, so that
𝑔1(𝑑, q1, q2) = 𝑓1(𝑑, q1, q2) = βˆ’,ΞΌXΞ·1q1 + pΞΌYΞ·2q2-
+
1
2
,(q1Ο‚X)2
+ p(q2Ο‚Y)2
+ pΟ‚XΟ‚YΞΌXΞΌYΞ³2
-(Ξ³ + v0)π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
Bringing Ξ΅βˆ—
= (Ο€βˆ—
, Ξ²βˆ—
, π‘ž1
βˆ—
, π‘ž2
βˆ—
) into the equation, after finishing, we get:
,βˆ’Ξ³π‘Ÿπ‘’π‘Ÿ(π‘‡βˆ’π‘‘)
+ Kt’- βˆ’ ,λμX(ΞΈ1 βˆ’ Ξ·1)
+Ξ»pΞΌY(ΞΈ2 βˆ’ Ξ·2)-Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
+ k(Ο‰ βˆ’ l)Kl +
1
2
Ο‚2
l(Kll + Kl
2
)
βˆ’Ξ»Ξ³er(Tβˆ’t)
𝑔1(q1
βˆ—
, q2
βˆ—
, t) βˆ’ g2(Ο€βˆ—
, t) βˆ’ g3(Ξ²βˆ—
, t) = 0#(24)
Where
Robust Optimal Reinsurance and Investment Problem with p-Thinning Dependent and Default ris
International Journal of Business Marketing and Management (IJBMM) Page 98
𝑔2(Ο€βˆ—
, t) = Ο€βˆ—
π‘™π‘’π‘Ÿ(π‘‡βˆ’π‘‘)(ςρ𝐾𝑙(Ξ³ + v1) βˆ’ 𝛼) +
1
2
(Ξ³ + v1)lΟ€βˆ—2
𝑒2π‘Ÿ(π‘‡βˆ’π‘‘) (25)
𝑔3(Ξ²βˆ—
, t) = βˆ’
(Ξ¦3 βˆ’ 1)Ξ³hp
v3
βˆ’ δβγer(Tβˆ’t)
#(26)
The derivative of equation (26) with respect to β :
Ξ²βˆ—
=
ln
1
βˆ†Ξ¦3
+ K(t, l) βˆ’ G(t, l)
ΞΆΞ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
#(27)
Bringing it into Ξ¦3
βˆ—
get :
hp
v3
Ξ¦3(𝑑) ln Ξ¦3(𝑑) + hp
Ξ¦3(𝑑) βˆ’
𝛿
𝜁
= 0. The equation has dimension one positive roots
Ξ¦3.
Let K(t, l)=K1(t)l + K2(t),satisfy the boundary conditions K1(T)=0,K2(T) = 0,bring it to HJB equation(24),
eliminating the effect of l on the equation yields two equations:
K1
’
βˆ’ k𝐾1 +
1
2
Ο‚2
𝐾1
2
βˆ’
𝐾1 βˆ’ 𝐺1
ΞΆ
Ξ΄ βˆ’ (ςρ𝐾1 + Ξ±(Ξ΄ βˆ’ r))Ξ±(Ξ΄ βˆ’ r)
+
1
2
(ςρ𝐾1 + Ξ±(Ξ΄ βˆ’ r))
2
βˆ’ (ςρ𝐾1 + Ξ±(Ξ΄ βˆ’ r))ςρ𝐾1 = 0#(28)
K2
’
βˆ’ ,λμX(ΞΈ1 βˆ’ Ξ·1) + Ξ»pΞΌY(ΞΈ2 βˆ’ Ξ·2)-Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
+ Ξ»pΟ‚XΟ‚YΞ³2
(π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
)2
+kω𝐾1 βˆ’
ln
1
βˆ†Ξ¦3
+ 𝐾2 βˆ’ 𝐺2
ΞΆ
Ξ΄ + (1 βˆ’
1
βˆ†Ξ¦3
) hp
βˆ’ f(t, π‘ž1
βˆ—
, π‘ž2
βˆ—) = 0#(29)
When ρ β‰  Β±1,equation (28) is the first-order RICCATI equation that,from the existence uniqueness of the
solution 𝐾1 = 𝐺1.Then we solve equation(29):let I(t)=𝐾2 βˆ’ 𝐺2, Satisfying the boundary condition I(T) = 0, then
using (29) and (21) we get:I’ = K2
’
βˆ’ G2
’
=
Ξ΄
ΞΆ
I +
Ξ΄
ΞΆ
ln
1
βˆ†Ξ¦3
+
Ξ³(Ξ¦3βˆ’1)
v3
hp
,
I = (ln
1
βˆ†Ξ¦3
+ βˆ† βˆ’ 1)𝑒
βˆ’
Ξ΄
ΞΆ
(Tβˆ’t)
βˆ’ ln
1
βˆ†Ξ¦3
βˆ’ βˆ† + 1.Thus :
𝐾2 = (ln
1
βˆ†Ξ¦3
+
Ξ³βˆ†(Ξ¦3 βˆ’ 1)
v3
) 𝑒
βˆ’
Ξ΄
ΞΆ
(Tβˆ’t)
βˆ’ ln
1
βˆ†Ξ¦3
+
Ξ³βˆ†(Ξ¦3 βˆ’ 1)
v3
+ 𝐺2(30)
Theorem III.3 The optimal pre-default investment and bond investment strategies are:
Ο€βˆ—
=
Ξ± βˆ’ ςρ(Ξ³ + v1)Kl
(Ξ³ + v1)π‘’π‘Ÿ(π‘‡βˆ’π‘‘)
, π›½βˆ—
=
𝑣3ln
1
ΔΦ3
+ Δ𝛾(Ξ¦3 βˆ’ 1)(𝑒
βˆ’
𝛿
𝜁
(Tβˆ’t)
βˆ’ 1)
𝑣3πœπ›Ύπ‘’π‘Ÿ(π‘‡βˆ’π‘‘)
The expression of the value function before default is:
𝑉(𝑑, π‘₯, 𝑙, 0) = βˆ’
1
𝛾
exp{βˆ’π›Ύπ‘’π‘Ÿ(π‘‡βˆ’π‘‘)
π‘₯ + K1(𝑑)𝑙+K2(𝑑)+
Where K1(t) = G1(t),as the formula (14)-(17),K2(t) as the formula (30).
IV. Parameter Sensitivity analysis
In this chapter, we give several numerical examples to test the effect of model parameters on the
optimal strategy. First, to make the parameters more realistic, the model parameters for credit bonds are set as
follows, based on the estimates from Berndt (2008)[22]
and Collin-Dufrensn & Solnik (2001)[23],
which are
estimated from market data:1/Ξ” = 2.53, hQ
= 0.013, 𝜁 = 0.52.The claim amounts for the first and second
categories of insurance respectively meet the parameters of λ𝑋 = 1.5, Ξ»π‘Œ = 1exponential distribution . If no
special statement is made, other model parameters are assumed as follows:: t = 3, T = 10, π‘Ÿ = 0.06, 𝛾 =
4,η1 = 2, Ξ·2 = 3, p = 0.5, 𝜈0 = 𝜈1 = 𝜈3 = 1, ρ = 1, Ο‚ = 0.16, π‘˜ = 2, Ξ± = 1.5.
1.7 Influence of parameters on optimal reinsurance strategy
Figure.1-2 represents the variation of the optimal reinsurance strategy with the parameters. where Figure.1
shows the variation of reinsurance strategy with probability p. When p increases, the insurer will reduce the
amount of reinsurance retention for class I reinsurance and increase the amount of reinsurance retention for class
II reinsurance. figure.2 represents the effect of risk aversion coefficient 𝛾 on reinsurance strategy, when the risk
aversion coefficient is larger, the insurer will reduce the reinsurance strategy and purchase more reinsurance to
diversify Claims risk.
Robust Optimal Reinsurance and Investment Problem with p-Thinning Dependent and Default ris
International Journal of Business Marketing and Management (IJBMM) Page 99
Figure. 1 Figure. 2
1.8 Influence of parameters on optimal investment strategy
Where Figure.3 represents the effect of the risk-free rate r on the optimal investment strategyΟ€βˆ—
, when the risk-
free rate increases, the insurer will invest less in risky assets and more assets in risk-free assets. Figure.4
represents the effect of the volatile reversion rate k on the optimal investment strategyΟ€βˆ—
. When the reversion
rate increases, the insurer will increase its investment in risky assets.
Figure. 3 Figure. 4
Figure.5 shows the impact of risk premium
1
Ξ”
on credit bonds, when the risk premium increases, investment in
credit bonds is increased. Figure.6 indicates that when the default loss rate 𝜁 increases, insurers will invest less
in credit bond.
Figure. 5 Figure. 6
1.9 Influence of ambiguity aversion coefficient on strategy
Figure.7 to Figure.9 represent the effect of optimal reinsurance-investment strategy subject to ambiguity
aversion sparsity. It can be seen that as the ambiguity aversion coefficient increases, the insurer's uncertainty
about the model increases and therefore reduces its optimal reinsurance-investment strategy.
0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6
p
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
q1
q2
2 2.5 3 3.5 4 4.5 5 5.5 6
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
q1
q2
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
r
0.05
0.1
0.15
0.2
0.25
0.3
1 1.5 2 2.5 3 3.5 4 4.5 5
k
0.222
0.2225
0.223
0.2235
0.224
0.2245
0.225
0.2255
0.226
1 2 3 4 5 6 7 8
1/
0
0.5
1
1.5
2
2.5
3
3.5
4
1
, =1.5
2
, =2.53
3
, =5
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
0.5
1
1.5
2
2.5
3
3.5
1
,1/ =1.5
2
,1/ =2.53
3
,1/ =5
Robust Optimal Reinsurance and Investment Problem with p-Thinning Dependent and Default ris
International Journal of Business Marketing and Management (IJBMM) Page 100
Figure. 7 Figure. 8
Figure. 9
V. Conclusion
This paper assumes that the insurer owns two types of insurance business with sparse dependence risk,
and the claim process is described by a diffusion approximation model, and secondly, the insurer expands its
investment types by investing in the financial market with a stock, a risk-free asset and a credit bond, with the
stock price described by a Heston model and the credit bond price described by an approximate model. The
credit bond price is described by the approximate model, and considering that the insurer is ambiguous averse,
the robust optimal reinsurance-investment problem is established, and the explicit expressions of the robust
optimal reinsurance-investment and optimal value function are obtained by using stochastic control theory,
dynamic programming principle, and HJB equation, and sensitivity analysis is performed on the model
parameters. Based on this paper, further discussions can be made: 1) other situations of the claim process can be
considered: such as jump diffusion or common shock. 2) time lag effects can be considered. 3) game problems
can be considered.
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0 1 2 3 4 5 6 7 8 9 10
0
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0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
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Robust Optimal Reinsurance and Investment Problem with p-Thinning Dependent and Default risks

  • 1. International Journal of Business Marketing and Management (IJBMM) Volume 8 Issue 2 Mar-Apr 2023, P.P. 91-101 ISSN: 2456-4559 www.ijbmm.com International Journal of Business Marketing and Management (IJBMM) Page 91 Robust Optimal Reinsurance and Investment Problem with p-Thinning Dependent and Default risks Yingqi Liang1 , Peibiao Zhao1 1 School of Mathematics and Statistics,Nanjing University of Science and Technology, Nanjing Jiangsu 210094,China 843186823@ Abstract: In this paper, we consider an AAI with two types of insurance business with p-thinning dependent claims risk, diversify claims risk by purchasing proportional reinsurance, and invest in a stock with Heston model price process, a risk-free bond, and a credit bond in the financial market with the objective of maximizing the expectation of the terminal wealth index effect, and construct the wealth process of AAI as well as the the model of robust optimal reinsurance-investment problem is obtained, using dynamic programming, the HJB equation to obtain the pre-default and post-default reinsurance-investment strategies and the display expression of the value function, respectively, and the sensitivity of the model parameters is analyzed through numerical experiments to obtain a realistic economic interpretation. The model as well as the results in this paper are a generalization and extension of the results of existing studies. Key words: p-Thinning dependent risk, defaultable bonds, dynamic programming, HJB equation, robust optimization. I. Introduction The sound operation and solid claiming ability of insurance companies are important to maintain social stability and promote social development, and the purchase of reinsurance can well control the risk of insurance companies. The current research on reinsurance in the actuarial field of insurance mainly focuses on the insurer's objective function, investment type, claim process, and price process of assets. [1]-[3]studied the most available reinsurance investment problems under different utility function. [4]-[7]investigate the optimal reinsurance investment problem under different risky asset price process. Most of the current literature considers only single-risk claims, while in reality the risks are usually correlated with each other. The current literature on claim dependency risk is divided into two main types of dependency, one is co-impact and the other is p-thinning dependency. P-thinning dependency means that when one type of claim occurs, there is a certain probability p that another type of claim will occur, for example, a car accident or fire will not only cause property damage, but also loss of life if the situation is serious. The problem under diffusion approximation model, jump diffusion risk model is studied in[8][9],[10] studied the type of dependent risk for common shocks, and [11] considered the risk process under p-thinning dependence. Most of the current studies consider only two assets for considering investments in credit bonds. [12]-[14] take into account defaulted bonds among the types of investments for insurers. Since models with diffusion and jump terms usually introduce uncertainty in the model for insurers, insurers usually want to seek a more robust model. [15]-[17] study the AAI most reinsurance-investment problem. Based on this, this paper studies the optimal proportional reinsurance-investment problem of AAI under the Heston model by considering both sparse dependence and claims risk based on [11][14][17]. The paper is organized as follows: a robust optimal reinsurance-investment problem model under p-thinning dependence and default risk is developed in Section 2. The value function is divided into pre-default and post-default in Section 3, and the optimal reinsurance-investment strategies are solved for pre-default and post-default using dynamic programming, optimal control theory, and the HJB equation, respectively. Parameter sensitivities are analyzed, and economic explanations are given in Section 4. Section 5 concludes the paper. II. Model Formulation Suppose *Ξ©, F, *𝐹𝑑+t∈,0,T -, P+ is a complete probability space, the positive number T denotes the final value moment, [0, T ] is a fixed time interval, 𝐹𝑑 denotes the information in the market up to time t, and 𝔽 ∢= *𝐹𝑑+t∈,0,T -denotes the standard Brownian motion π‘Š0(𝑑), π‘Š1(𝑑), π‘Š2(𝑑), π‘Š3(𝑑). Poisson process N(t), and the right-continuous P-complete information flow generated by the sequence of random variables *𝑋𝑖, 𝑖 β‰₯ 1+, *π‘Œπ‘–, 𝑖 β‰₯ 1+. ℍ ∢= (ℋ𝑑)tβ©Ύ0 is the information flow generated by the violation process H(t), let 𝔾: = (𝒒𝑑)tβ©Ύ0, be the information flow generated by 𝔽, ℍ the expanded information flow, i.e., 𝔾: = ℱ𝑑 ∨ ℋ𝑑. By definition, each 𝔽 -harness is also the 𝔾 -harness. The probability measure P is a realistic probability measure and Q is a risk- neutral measure. In addition, it is assumed that all transactions in the financial market are continuous, and no
  • 2. Robust Optimal Reinsurance and Investment Problem with p-Thinning Dependent and Default ris International Journal of Business Marketing and Management (IJBMM) Page 92 taxes do not incur transaction costs and all property is infinitely divisible. 1.1 Surplus Process Assuming that the insurer operates two different lines of business and that there is a sparse dependency between these two lines of business, the surplus process is as follows: R(t) = π‘₯0 + 𝑐𝑑 βˆ’ (βˆ‘ 𝑋𝑖 𝑁(𝑑) 𝑖=1 + βˆ‘ π‘Œπ‘– 𝑁𝑝(𝑑) 𝑖=1 ) . #(1) where{π‘‹π‘–οΌŒπ‘– β‰₯ 1} is independently and identically distributed in 𝐹𝑋(βˆ™),𝐸(𝑋) = πœ‡π‘‹ > 0,𝐸(𝑋2 ) = πœπ‘‹ 2 , as the claim amount of the first class of business,{π‘Œπ‘–οΌŒπ‘– β‰₯ 1} is independently and identically distributed inπΉπ‘Œ(βˆ™ ),𝐸(π‘Œ) = πœ‡π‘Œ > 0,𝐸(π‘Œ2 ) = πœπ‘Œ 2 , as the claim amount of the second class of business. the claim amount of the second type of business. N(t)denotes the conforming Poisson process with parameter c denotes the premium of the insurance company by the expected value premium there are c = (1 + ΞΈ1)Ξ»πœ‡π‘‹ + (1 + ΞΈ2)Ξ»pπœ‡π‘Œ.ΞΈ1 > 0,ΞΈ2 > 0. 1.2 Proportional Reinsurance Assuming that the insurer diversifies the claim risk by purchasing proportional reinsurance, and let π‘ž1(𝑑),π‘ž2(𝑑) be the insurer's retention ratio, the claim after the insurer purchases reinsurance is: π‘ž1(𝑑)𝑋𝑖 , π‘ž2(𝑑)π‘Œπ‘– then the reinsurance fee is Ξ΄(π‘ž1(𝑑), π‘ž2(𝑑)) = (1 + Ξ·1)(1 βˆ’ π‘ž1(𝑑))Ξ»πœ‡π‘‹ + (1 + Ξ·2)(1 βˆ’ π‘ž2(𝑑))Ξ»pπœ‡π‘Œ.According to Grandell(1991)[8] ,the claims process can be diffusely approximated as: d βˆ‘ Xi N(t) i=1 = Ξ»E(Xi)dt βˆ’ Ξ³1dWX(t), Ξ³1 = √λE(XI 2 ) d βˆ‘ Yi Np(t) i=1 = Ξ»E(Yi)dt βˆ’ Ξ³2dWY(t), Ξ³2 = √λE(YI 2 ) The correlation coefficient of π‘Šπ‘‹(𝑑)π‘Šπ‘Œ(𝑑 )is ρ Μ‚ = Ξ»p 𝛾1𝛾2 E(𝑋𝑖)E(π‘Œπ‘–).Then the wealth process of the insurer after joining the reinsurance is: dXq1,q2 = ,λμX(ΞΈ1 βˆ’ Ξ·1 + Ξ·1π‘ž1(𝑑)) + Ξ»pΞΌY(ΞΈ2 βˆ’ Ξ·2 +Ξ·2π‘ž2(𝑑))𝑑𝑑 + √(π‘ž1𝛾1)2 + (π‘ž2𝛾2)2 + 2ρ Μ‚π‘ž1π‘ž2𝛾1𝛾2dW0 1.3 Financial Market Suppose the financial market consists of three assets: risk-free assets, stocks, and corporate bonds, and the price processes of the three assets are as follows: The price process of risk-free bonds is given byοΌšπ‘‘R(t) = rR(t)dt.The stock price process S(t) obeys the Heston stochastic volatility model: { 𝑑𝑆(𝑑) = 𝑆(𝑑),π‘Ÿ + 𝛼𝐿(𝑑)𝑑𝑑 + √𝐿(𝑑)π‘‘π‘Š1(𝑑)-, 𝑆(0) = 𝑠0 𝑑𝐿(𝑑) = π‘˜(πœ” βˆ’ 𝐿(𝑑))𝑑𝑑 + 𝜍√𝐿(𝑑)π‘‘π‘Š2(𝑑), 𝐿(0) = 𝑙0 R is the risk-free rate,Ξ±, π‘˜, Ο‚, are positive constant.𝐸,π‘Š1π‘Š2- = ρt, 2π‘˜πœ” β‰₯ Ο‚2 Following Bielecki (2007)[18] , the credit bond price process p(t, T1), under a realistic measure P, using an approximate model to portray default risk is as follows: dp(t, T1) = p(tβˆ’, T1),(r + (1 βˆ’ H(t))Ξ΄(1 βˆ’ Ξ”)dt βˆ’(1 βˆ’ H(tβˆ’))ΞΆdMp (t)- Where Mp (t) = H(t) βˆ’ hQ ∫ Ξ”(1 βˆ’ H(u))du t 0 is aβ„Š βˆ’ harness, Ξ΄ = 𝑕𝑄 ΞΆ is the credit spread. 1.4 Robust optimization problem Assuming the insurer adopts a reinsurance investment strategyΞ΅(t) = (π‘ž1(𝑑), π‘ž2(𝑑), Ο€(t), Ξ²(𝑑))οΌŒπ‘ž1(𝑑), π‘ž2(𝑑)are the reinsurance strategies adopted by the insurer at moment t in the first and second asset classes, respectively.Ο€(t) for the insurance company's investment in equities at time t.,β(𝑑)for the insurance companyβ€²s investment in credit bonds at time t. Let 𝔸 denotes the set of all feasible strategies, then the dynamic process of wealth of the insurance company at this moment π‘‹πœ€ (t) is: dπ‘‹πœ€(𝑑) = πœ‹(𝑑) 𝑑𝑆(𝑑) 𝑆(𝑑) + Ξ²(𝑑) 𝑑𝑝(𝑑) 𝑝(𝑑) + (π‘‹πœ€(𝑑) βˆ’ πœ‹(𝑑) βˆ’ Ξ²(𝑑)) 𝑑𝐡(𝑑) 𝐡(𝑑)
  • 3. Robust Optimal Reinsurance and Investment Problem with p-Thinning Dependent and Default ris International Journal of Business Marketing and Management (IJBMM) Page 93 +π‘‘π‘‹π‘š(𝑑),XΞ΅(0) = x0 Assume that the insurer maximizes the terminal T moment expected utility in the financial market, the portfolio index utility, which takes the form of: V(X(T)) = βˆ’ 1 𝛾 π‘’βˆ’π›Ύπ‘‹(𝑇) Where 𝛾 > 0 is the ambiguity aversion coefficient of the insurer. The insurer's goal is to find the optimal reinsurance-investment strategyπœ€βˆ— (𝑑) = (π‘ž1 βˆ— (𝑑), π‘ž2 βˆ— (𝑑), πœ‹βˆ— (𝑑), Ξ²βˆ— (𝑑))to maximizing the expectation of end-use wealth utility for insurers. The objective function of the insurer is: VΞ΅ (t, x, l, h) = E(u(XΞ΅ (T))|XΞ΅ (t) = x, L(t) = l) The value function of the optimization problem is: V(t, x, l, h) = VΞ΅ (t, x, l, h) Ρ∈Π sup , V(T,x,l,h)=v(x), Assume that the ambiguity information is described by the probability P and the reference model is measured by the probability 𝒫Φ which is equivalent to PοΌšπ’«: = *𝒫Φ |𝒫Φ ~𝑃+.Next, the optional measure set is constructed, defining the procedure :Ξ¦(𝑑) = (Ξ¦0(𝑑), Ξ¦1(𝑑), Ξ¦2(𝑑), Ξ¦3(𝑑))s.t.: 1. Ξ¦(𝑑) is a β„Š(𝑑)- measurable for any t[0,T] 2. Ξ¦i(t) = Ξ¦i(t, Ο‰), i = 0,1,2,3 andΞ¦i(t) β‰₯ 0 for all (t, Ο‰) ∈ ,0, T- Γ— Ξ©. 3.∫ ||Ξ¦(t)||2 𝑑𝑑 < ∞; 𝑇 0 Let Ξ£ be all processes shaped as Φ,for all Ξ¦ ∈ Ξ£ define a G-adaptation process under a real measure *ΛΦ (𝑑)|𝑑 ∈ ,0, T-+.From Ito differentiation we have: 𝑑ΛΦ (𝑑) = ΛΦ (π‘‘βˆ’)(βˆ’Ξ¦0(𝑑)dW0 βˆ’ Ξ¦1(𝑑)π‘‘π‘Š1 βˆ’ Ξ¦2(𝑑)π‘‘π‘Š2 βˆ’ (1 βˆ’ Ξ¦3(𝑑))𝑑Mp ) Where ΛΦ (0) = 1,P-a.s. ΛΦ (𝑑) is a (P,G)- martingale,E[ΛΦ (𝑇)] =1, for each Ξ¦ ∈ Ξ£,A new optional measure is absolutely continuous to P, defined as 𝑑𝑃Φ 𝑑𝑃 |𝒒𝑇 = ΛΦ (𝑇). So far, we have constructed a class of real-world probability measures 𝑃Φ ,where Ξ¦ ∈ Σ,From Gisanova's theorem[21] ,it follows that: 𝑑Wi 𝑃Φ (𝑑) = dWi(𝑑) + Φ𝑖(𝑑)𝑑𝑑, 𝑖 = 0,1,2 Therefore the wealth process in the 𝑃Φ is: dπ‘‹πœ€ (𝑑) = ,xr + πœ‹(𝛼𝑙 βˆ’ Ξ¦1βˆšπ‘™) + Ξ²(1 βˆ’ H(t))Ξ΄(1 βˆ’ Ξ”) +λμX(ΞΈ1 βˆ’ Ξ·1) + Ξ»pΞΌY((ΞΈ2 βˆ’ Ξ·2) + Ξ·1π‘ž1(𝑑)) + Ξ·2π‘ž2(𝑑)) +√(π‘ž1𝛾1)2 + (π‘ž2𝛾2)2 + 2ρ Μ‚π‘ž1π‘ž2𝛾1𝛾2Ξ¦0-dt + πœ‹βˆšπ‘™π‘‘W1 𝑃Φ +Ξ²(1 βˆ’ H(tβˆ’))ΞΆdMp + √(π‘ž1𝛾1)2 + (π‘ž2𝛾2)2 + 2ρ Μ‚π‘ž1π‘ž2𝛾1𝛾2𝑑W0 𝑃Φ (2) We assume that the insurer determines a robust portfolio strategy such that the worst-case scenario is the best option. The insurer penalizes any deviation from the sub-reference model with a penalty that increases with this deviation, using relative entropy to measure the deviation between the reference measure and the optional measure. Inspired by Maenhout[19] Branger[20] the problem is modified to define the value function as: V(t, x, l. h) = 𝐸𝑑,π‘₯,𝑙,β„Ž 𝑃Φ 0βˆ’ 1 𝛾 π‘’βˆ’π›Ύπ‘‹(𝑇) + ∫ 𝐺(𝑒, π‘‹πœ€(𝑒), πœ™(𝑒))𝑑𝑒 𝑇 𝑑 1 Φ∈Σ 𝑖𝑛𝑓 πœ€βˆˆπ›± 𝑠𝑒𝑝 𝐸𝑑,π‘₯,𝑙,β„Ž 𝑃Φ Calculated under the optional measure , the initial value of the stochastic process is π‘‹πœ€(𝑑)=x, S(t)=s, H(t)=h, G(u, XΞ΅(u), Ο•(u)) = Ξ¦0 2 2Ξ¨0(t, XΞ΅(t), Ο•(t)) + Ξ¦1 2 2Ξ¨1(t, XΞ΅(t), Ο•(t)) + Ξ¦2 2 2Ξ¨2(t, XΞ΅(t), Ο•(t)) + (Ξ¦3 ln Ξ¦3 βˆ’ Ξ¦3 + 1)hp (1 βˆ’ h) 2Ξ¨3(t, XΞ΅(t), Ο•(t)) Where Ξ¨0 β‰₯ 0,Ψ1 β‰₯ 0,Ψ2 β‰₯ 0,Ψ3 β‰₯ 0 is state-dependent,let Ξ¨0 = βˆ’ 𝑣0 𝛾V(t,x,l,h) , Ξ¨1 = βˆ’ 𝑣1 𝛾V(t,x,l,h) , Ξ¨2 = βˆ’ 𝑣2 𝛾V(t,x,l,h) , Ξ¨3 = βˆ’ 𝑣3 𝛾V(t,x,l,h) 𝑣𝑖,i=0,1,2,3 is the risk aversion factor, the lager Ξ¨i. According to the dynamic planning principle, the HJB equation is as follows: π’œπœ€,πœ™ 𝑉 + 𝐺(𝑒, π‘‹πœ€(𝑒), πœ™(𝑒)) = 0 Ξ¦πœ–Ξ£ 𝑖𝑛𝑓 πœ€πœ–βˆ 𝑠𝑒𝑝 #(3) π’œπœ€,πœ™ is the infinitesimal operator under the measure 𝑃Φ .
  • 4. Robust Optimal Reinsurance and Investment Problem with p-Thinning Dependent and Default ris International Journal of Business Marketing and Management (IJBMM) Page 94 III. Robust optimal reinsurance-investment strategy solving This section will solve the robust optimal problem constructed in the previous section. This paper divides the value function into pre-default and post-default components according to the time of default of the credit bond: V(T, x, l, h) = { V(T, x, l, 0),h = 0(before default) V(T, x, l, 1),h = 1(after default) By decomposing the value function into two sub-functions, denoted as the value function before the zero- coupon bond default and the value function after the zero-coupon bond default, the two sub-HJB equations are obtained and solved successively to obtain the reinsurance and risky asset investment strategies and value function expressions after default, and the reinsurance, risky asset and credit bond investment strategies and value function expressions before default. 1.5 Optimal reinsurance and investment decisions after default When H (t)=1οΌŒΟ„ ∧ T ≀ t ≀ T,the insurer has constituted a default at or before time t, the HJB equation degenerates to: 𝑉𝑑 + [π‘Ÿπ‘₯ + πœ‹(𝛼𝑙 βˆ’ Ξ¦1βˆšπ‘™) + πœ†ΞΌX(ΞΈ1 βˆ’ Ξ·1) + Ξ»pΞΌY(ΞΈ2 βˆ’ Ξ·2) + πœ†ΞΌXΞ·1π‘ž1(𝑑) + Ξ»pΞΌYΞ·2π‘ž2(𝑑) + √(π‘ž1𝛾1)2 + (π‘ž2𝛾2)2 + 2ρ Μ‚π‘ž1π‘ž2𝛾1𝛾2Ξ¦0]𝑉 π‘₯ + 1 2 (Ο€2 l + Ξ»(q1Ο‚X)2 + Ξ»p(q2Ο‚Y)2 + 2Ξ»pΞΌXΞΌY)Vxx + Ο€lςρVxl + (k(Ο‰ βˆ’ l) βˆ’ Ξ¦1πœπœŒβˆšπ‘™ βˆ’ Ξ¦2𝜍√1 βˆ’ 𝜌2βˆšπ‘™)Vl + 1 2 Ο‚2 lVll βˆ’ Ξ¦0 2 2v0 𝛾 βˆ’ Ξ¦1 2 2v1 𝛾 βˆ’ Ξ¦2 2 2v2 𝛾 = 0 (4) Satisfied :V(T, x, l, 1) = βˆ’ 1 Ξ³ eβˆ’Ξ³X The solution can be assumed to be of the form: V(t, x, y, l, 1) = βˆ’ 1 Ξ³ exp*βˆ’Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘) x + G(t, l)+, G(T, l) = 0#(5) Taking each partial derivative of V: { 𝑉𝑑 = [Ξ³rxπ‘’π‘Ÿ(π‘‡βˆ’π‘‘) + Gt]V, 𝑉 π‘₯ = βˆ’Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘) V 𝑉 π‘₯π‘₯ = Ξ³2 𝑒2π‘Ÿ(π‘‡βˆ’π‘‘) π‘‰οΌŒπ‘‰π‘₯𝑙 = βˆ’Ξ³πΊπ‘™π‘‰ 𝑉𝑙 = 𝐺𝑙𝑉, 𝑉𝑙𝑙 = 𝐺𝑙𝑙𝑉 + 𝐺𝑙 2 𝑉 The minimum value point of Ξ¦βˆ— is obtained from the first order condition as: { Ξ¦0 βˆ— = v0er(Tβˆ’t) √(q1Ξ³1)2 + (q2Ξ³2)2 + 2ρ Μ‚q1q2Ξ³1Ξ³2 Ξ¦1 βˆ— = v1er(Tβˆ’t) Ο€βˆšl, Ξ¦2 βˆ— = βˆ’Ο‚ΟβˆšlGlv2 1 Ξ³ Bringing in the HJB equation yields: Ξ³rxπ‘’π‘Ÿ(π‘‡βˆ’π‘‘) + Gt βˆ’ ,λμX(ΞΈ1 βˆ’ Ξ·1) + Ξ»pΞΌY(ΞΈ2 βˆ’ Ξ·2)- Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘) + k(Ο‰ βˆ’ l)𝐺𝑙 + (πœπœŒβˆšπ‘™πΊπ‘™) 2 v2 2 + 1 2 𝜍2 𝑙(𝐺𝑙𝑙 + 𝐺𝑙 2 ) + *𝑓2(πœ‹, 𝑑)+ Ο€ inf + {Ξ»Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘) 𝑓1(π‘ž1, π‘ž2, 𝑑)} π‘ž1,π‘ž2 inf = 0#(6) Where 𝑓1(π‘ž1, π‘ž2, 𝑑) = βˆ’,ΞΌXΞ·1π‘ž1(𝑑) + pΞΌYΞ·2π‘ž2(𝑑)- + (Ξ³ + v0)π‘’π‘Ÿ(π‘‡βˆ’π‘‘) 2 ,(π‘ž1πœπ‘‹)2 + p(π‘ž2πœπ‘Œ)2 + 2pΟ‚XΟ‚Yπ‘ž1π‘ž2- 𝑓2(πœ‹, 𝑑) = πœ‹π‘™π‘’π‘Ÿ(π‘‡βˆ’π‘‘)(ςρ𝐺𝑙(Ξ³ + v1) βˆ’ 𝛼) + 1 2 (Ξ³ + v1)lΟ€2 𝑒2π‘Ÿ(π‘‡βˆ’π‘‘) Theorem III.1 Let m= ΞΌX(Ξ·1πœŽπ‘Œ 2βˆ’pΞ·2ΞΌY 2) 𝜎π‘₯ 2πœŽπ‘Œ 2βˆ’π‘ΞΌX 2ΞΌY 2 , n= ΞΌY(Ξ·2πœŽπ‘‹ 2βˆ’Ξ·1ΞΌX 2) 𝜎π‘₯ 2πœŽπ‘Œ 2βˆ’π‘ΞΌX 2ΞΌY 2 ,then there exist 𝑑1, 𝑑2𝑑1 Μ‚, 𝑑2 Μ‚ and the following values can be obtained:
  • 5. Robust Optimal Reinsurance and Investment Problem with p-Thinning Dependent and Default ris International Journal of Business Marketing and Management (IJBMM) Page 95 𝑑1 = 𝑇 βˆ’ 1 π‘Ÿ ln π‘š Ξ³ + v0 , 𝑑2 = 𝑇 βˆ’ 1 π‘Ÿ ln 𝑛 Ξ³ + v0 𝑑1 Μ‚ = 𝑇 βˆ’ 1 π‘Ÿ ln ΞΌXΞ·1 (Ξ³ + v0)p(πœπ‘‹ 2 + ΞΌXΞΌY) 𝑑2 Μ‚ = 𝑇 βˆ’ 1 π‘Ÿ ln ΞΌyΞ·2 (Ξ³ + v0)(πœπ‘Œ 2 + ΞΌXΞΌY) When m ≀ Ξ³(n ≀ Ξ³),let 𝑑2 = 𝑇(𝑑1 = 𝑇).When m > Ξ³(n > Ξ³),let 𝑑2 = 0(𝑑1 = 0)(1)If π‘š ≀ n,then π‘ž1 βˆ— ≀ π‘ž2 βˆ— ,for all tΟ΅,0, T-,The reinsurance strategy corresponding to the problem is(π‘ž1 βˆ— , π‘ž2 βˆ— ) = { (π‘ž1 Μ‚ , π‘ž2 Μ‚),0 ≀ 𝑑 ≀ 𝑑2 (π‘ž1 Μƒ, 1 Μƒ)οΌŒπ‘‘2 ≀ 𝑑 ≀ 𝑑1 Μ‚ (1,1)οΌŒπ‘‘1 ≀ 𝑑 ≀ 𝑇 (2)If n > m,then for alltΟ΅,0, T-,The reinsurance strategy corresponding to the problem is(π‘ž1 βˆ— , π‘ž2 βˆ— ) = { (π‘ž1 Μ‚ , π‘ž2 Μ‚),0 ≀ 𝑑 ≀ 𝑑1 (1, π‘ž2 Μƒ), 𝑑1 ≀ 𝑑 ≀ 𝑑2 Μ‚ (1,1), 𝑑2 Μ‚ ≀ 𝑑 ≀ 𝑇 . Where π‘ž1 Μ‚ = ΞΌX(Ξ·1πœπ‘Œ 2 βˆ’ pΞ·2ΞΌY 2) (Ξ³ + v0)π‘’π‘Ÿ(π‘‡βˆ’π‘‘)(𝜍π‘₯ 2πœπ‘Œ 2 βˆ’ 𝑝μX 2ΞΌY 2) #(7) π‘ž2 Μ‚ = ΞΌY(Ξ·2πœπ‘‹ 2 βˆ’ Ξ·1ΞΌX 2) (Ξ³ + v0)π‘’π‘Ÿ(π‘‡βˆ’π‘‘)(𝜍π‘₯ 2πœπ‘Œ 2 βˆ’ 𝑝μX 2ΞΌY 2) #(8) π‘ž1 Μƒ = ΞΌXΞ·1 βˆ’ pΞΌXΞΌY(Ξ³ + v0)π‘’π‘Ÿ(π‘‡βˆ’π‘‘) πœπ‘‹ 2(Ξ³ + v0)π‘’π‘Ÿ(π‘‡βˆ’π‘‘)p #(9) π‘ž2 Μƒ = ΞΌyΞ·2 βˆ’ ΞΌXΞΌY(Ξ³ + v0)π‘’π‘Ÿ(π‘‡βˆ’π‘‘) πœπ‘Œ 2(Ξ³ + v0)π‘’π‘Ÿ(π‘‡βˆ’π‘‘) #(10) Proof:By finding the first-order partial derivatives, second-order partial derivatives and second-order mixed partial derivatives for𝑓1, the following system of equations and the Hessian array are obtained: { πœ•π‘“1 πœ•π‘ž1 = βˆ’ΞΌXΞ·1 + (Ξ³ + v0)π‘’π‘Ÿ(π‘‡βˆ’π‘‘) ,πœπ‘‹ 2 π‘ž1 + pΟ‚XΟ‚Yπ‘ž2- = 0 πœ•π‘“1 πœ•π‘ž2 = βˆ’ΞΌYΞ·2 + (Ξ³ + v0)π‘’π‘Ÿ(π‘‡βˆ’π‘‘) ,πœπ‘Œ 2 π‘ž2 + Ο‚XΟ‚Yπ‘ž1- = 0 (11) | | πœ•2 𝑓1 πœ•π‘ž1πœ•π‘ž1 πœ•2 𝑓1 πœ•π‘ž1πœ•π‘ž2 πœ•2 𝑓1 πœ•π‘ž2πœ•π‘ž1 πœ•2 𝑓1 πœ•π‘ž2πœ•π‘ž2 | | = 𝑒4π‘Ÿ(π‘‡βˆ’π‘‘) (Ξ³ + v0)2 | βˆ’πœπ‘₯ 2 (Ξ³ + v0) βˆ’pΟ‚XΟ‚Y βˆ’pΟ‚XΟ‚Y βˆ’π‘πœπ‘¦ 2 (Ξ³ + v0) | From the Hessian array positive definite it is known that 𝑓1(π‘ž1, π‘ž2, 𝑑) is a convex function and there exist extreme value points; solving the system of equations (11) yields (7). (8). ObviouslyοΌŒπ‘ž1 Μ‚ γ€π‘ž2 Μ‚ are both increasing functions with respect to t, when π‘š ≀ n,0 ≀ 𝑑 ≀ 𝑑1 or n > m,0 ≀ 𝑑 ≀ 𝑑2the solution as (7)(8),Also the values of 𝑑1 and 𝑑2 can be found. When π‘š ≀ nοΌŒπ‘‘2 ≀ 𝑑 ≀ 𝑑1 Μ‚ ,then (q1 βˆ— , q2 βˆ— ) = (π‘ž1 Μƒ, 1),q1 ∈ ,0,1-.When 𝑛 > mοΌŒπ‘‘1 ≀ 𝑑 ≀ 𝑑2 Μ‚ then (q1 βˆ— , q2 βˆ— ) = (1, π‘ž1 Μƒ), q2 ∈ ,0,1-,Separate solve df1(t,q1,1) dq1 = 0, df1(t,1,q2) dq2 = 0 , then can get π‘ž1 Μƒ, π‘ž2 Μƒ as (9), (10) End. Theorem III.2 The post-default insurer's optimal risky asset investment strategy is to Ο€βˆ— = Ξ± βˆ’ ςρ(Ξ³ + v1)𝐺1 (Ξ³ + v1)π‘’π‘Ÿ(π‘‡βˆ’π‘‘) #(12) The expression of the optimal value function is: V(t, x, y, l, 1) = βˆ’ 1 Ξ³ exp*βˆ’Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘)(t)x + G1(t)l + G2(t)+#(13) π‘€π‘•π‘’π‘Ÿπ‘’ ρ β‰  Β±1, 𝐺1 = 𝑙1𝑙2 βˆ’ 𝑙1𝑙2π‘’βˆ’ 1 2 Ο‚2(𝑙1βˆ’π‘™2)(Ξ³+v1)(1βˆ’Ο2)(π‘‡βˆ’π‘‘) 𝑙2 βˆ’ 𝑙1π‘’βˆ’ 1 2 Ο‚2(𝑙1βˆ’π‘™2)(Ξ³+v1)(1βˆ’Ο2)(π‘‡βˆ’π‘‘) , #(14) ρ = 1, 𝐺1 = Ξ±2 2(Ξ³ + v1)(Ξ±Ο‚ + k) (1 βˆ’ π‘’βˆ’(Ξ±Ο‚+k)(π‘‡βˆ’π‘‘) ), #(15)
  • 6. Robust Optimal Reinsurance and Investment Problem with p-Thinning Dependent and Default ris International Journal of Business Marketing and Management (IJBMM) Page 96 ρ = βˆ’1, k β‰  Ξ±Ο‚, 𝐺1 = Ξ±2 2(Ξ³ + v1)(π‘˜ βˆ’ Ξ±Ο‚) (1 βˆ’ 𝑒(Ξ±Ο‚βˆ’k)(π‘‡βˆ’π‘‘) ), #(16) ρ = βˆ’1, π‘˜ = Ξ±Ο‚, 𝐺1 = Ξ±2 2(Ξ³ + v1) , #(17) 𝑙1,2 = βˆ’(αςρ + k) Β± √(αςρ + k)2 + Ξ±2Ο‚2(1 βˆ’ ρ2) Ξ³Ο‚2(1 βˆ’ ρ2) G2 = Ξ³ r ,Ξ»(ΞΌX(ΞΈ1 βˆ’ Ξ·1) + Ξ»p(ΞΌY(ΞΈ2 βˆ’ Ξ·2)-,1 βˆ’ π‘’π‘Ÿ(π‘‡βˆ’π‘‘) - +π‘˜πœ” ∫ G1(s)ds 𝑇 𝑑 + ∫ Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘) 𝑓1(π‘ž1 βˆ— , π‘ž2 βˆ— , 𝑠)𝑑𝑠 𝑇 𝑑 #(18) Proof: Let G(t, l) = G1(t)l + G2(t)#(19) Bringing equation (19) into equation (6) can be obtained as the following two equations: G1 ’ βˆ’ Ξ±2 2(Ξ³ + v1) βˆ’ G1(αςρ + k) + 1 2 (Ξ³ + v1)Ο‚2 G1 2 (ρ2 βˆ’ 1) = 0#(20) G2β€˜ βˆ’ ,Ξ»(ΞΌX(ΞΈ1 βˆ’ Ξ·1) + Ξ»p(ΞΌY(ΞΈ2 βˆ’ Ξ·2)-Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘) +π‘˜πœ”G1 + Ξ»Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘) 𝑓1(π‘ž1 βˆ— , π‘ž2 βˆ— , 𝑑) = 0#(21) Solving equation (20) yields equations (14)-(17). From equation (21) we get (18), and the specific expression of (18) is discussed in the following cases: 1)When π‘š ≀ n,0 ≀ 𝑑 ≀ 𝑑2(n > m, 0 ≀ 𝑑 ≀ 𝑑1) G2 = Ξ³ π‘Ÿ ,Ξ»(ΞΌX(ΞΈ1 βˆ’ Ξ·1) + Ξ»p(ΞΌY(ΞΈ2 βˆ’ Ξ·2)-,π‘’π‘Ÿ(π‘‡βˆ’π‘‘) βˆ’ 1- + G1(t) Μ‚ βˆ’Ξ»(T βˆ’ t)*ΞΌXΞ·1 ΞΌX(Ξ·1πœπ‘Œ 2 βˆ’ pΞ·2ΞΌY 2) (𝜍π‘₯ 2πœπ‘Œ 2 βˆ’ 𝑝μX 2ΞΌY 2) + pΞΌYΞ·2 ΞΌY(Ξ·2πœπ‘‹ 2 βˆ’ pΞ·1ΞΌX 2) (𝜍π‘₯ 2πœπ‘Œ 2 βˆ’ 𝑝μX 2ΞΌY 2) βˆ’ 1 2 ,( ΞΌXπœπ‘‹(Ξ·1πœπ‘Œ 2 βˆ’ pΞ·2ΞΌY 2) (𝜍π‘₯ 2πœπ‘Œ 2 βˆ’ 𝑝μX 2ΞΌY 2) ) 2 + p ( ΞΌYπœπ‘Œ(Ξ·2πœπ‘‹ 2 βˆ’ pΞ·1ΞΌX 2) (𝜍π‘₯ 2πœπ‘Œ 2 βˆ’ 𝑝μX 2ΞΌY 2) ) 2 +2pΟ‚XΟ‚Y ΞΌX(Ξ·1πœπ‘Œ 2 βˆ’ pΞ·2ΞΌY 2 ) (𝜍π‘₯ 2πœπ‘Œ 2 βˆ’ 𝑝μX 2ΞΌY 2) ΞΌY(Ξ·2πœπ‘‹ 2 βˆ’ pΞ·1ΞΌX 2 ) (𝜍π‘₯ 2πœπ‘Œ 2 βˆ’ 𝑝μX 2ΞΌY 2) -+ 2οΌ‰When π‘š ≀ 𝑛, 𝑑2 ≀ 𝑑 ≀ 𝑑1 Μ‚, G2 = Ξ³ r ,Ξ»(ΞΌX(ΞΈ1 βˆ’ Ξ·1) + Ξ»p(ΞΌY(ΞΈ2 βˆ’ Ξ·2)-,π‘’π‘Ÿ(π‘‡βˆ’π‘‘) βˆ’ 1- + πœ†π›Ύ (𝛾 + v0) ( ΞΌXΞ·1 πœπ‘‹π‘ ) 2 ( 1 2 βˆ’ p)(T βˆ’ t) + Ξ»(βˆ’ ΞΌXΞ·1pΞΌXΞΌY πœπ‘‹ 2p βˆ’Ξ»pΞΌYΞ·2 + ΞΌXΞ·1ΞΌXΞΌY πœπ‘‹ + Ο‚Y ΞΌXΞ·1 Ο‚X ) Ξ³ r (π‘’π‘Ÿ(π‘‡βˆ’π‘‘) βˆ’ 1) +,( ΞΌXΞΌY πœπ‘‹ ) 2 + pπœπ‘Œ 2 βˆ’ 2𝑝ςY ΞΌXΞΌY πœπ‘‹ - λγ(𝛾 + v0) 2r (𝑒2π‘Ÿ(π‘‡βˆ’π‘‘) βˆ’ 1) + π‘˜πœ”G1(t) Μ‚ 3οΌ‰When π‘š > 𝑛, 𝑑1 ≀ 𝑑 ≀ 𝑑2 Μ‚,optimal reinsurance strategy (π‘ž1 βˆ— , π‘ž2 βˆ— ) =(1,π‘ž2 Μƒ), G2 = Ξ³ r ,Ξ»(ΞΌX(ΞΈ1 βˆ’ Ξ·1) + Ξ»p(ΞΌY(ΞΈ2 βˆ’ Ξ·2)-,π‘’π‘Ÿ(π‘‡βˆ’π‘‘) βˆ’ 1- + λγ (Ξ³ + 𝜈0) ( ΞΌyΞ·2 πœπ‘Œ ) 2 ( 1 2p βˆ’ 1) (T βˆ’ t) + π‘˜πœ”G1(t) Μ‚ +λμyΞ·2 (ΞΌXΞ·1 βˆ’ 𝑝μYΞ·2 ΞΌXΞΌY πœπ‘Œ 2 βˆ’ Ο‚X ΞΌyΞ·2 πœπ‘Œ + ΞΌyΞ·2ΞΌXΞΌY 2πœπ‘Œ 2 ) Ξ³(π‘’π‘Ÿ(π‘‡βˆ’π‘‘) βˆ’ 1) r +(πœπ‘‹ 2 βˆ’ 2pΟ‚X ΞΌXΞΌY πœπ‘Œ + p( ΞΌXΞΌY πœπ‘Œ )2 ) λ𝛾(𝛾 + v0) 4r (𝑒2π‘Ÿ(π‘‡βˆ’π‘‘) βˆ’ 1) 4) When π‘š > 𝑛, 𝑑2 Μ‚ ≀ 𝑑 ≀ T(π‘š ≀ 𝑛, 𝑑1 Μ‚ ≀ 𝑑 ≀ 𝑇), optimal reinsurance strategy (π‘ž1 βˆ— , π‘ž2 βˆ— ) =(1,1), G2 = Ξ³ r ,λμXΞΈ1 + Ξ»pΞΌYΞΈ2-,π‘’π‘Ÿ(π‘‡βˆ’π‘‘) βˆ’ 1- +π‘˜πœ”G1(t) Μ‚ + λγ(Ξ³ + v0) 4 ,(πœπ‘‹)2 + p(πœπ‘Œ)2 + 2pΟ‚XΟ‚Y-(𝑒2π‘Ÿ(π‘‡βˆ’π‘‘) βˆ’ 1) Where G1(t) Μ‚ = kΟ‰ ∫ G1(s)ds T t = 𝑙2π‘˜πœ”(𝑇 βˆ’ 𝑑) βˆ’ 2π‘˜πœ”(𝜍2 (1 βˆ’ 𝜌2 )βˆ’1
  • 7. Robust Optimal Reinsurance and Investment Problem with p-Thinning Dependent and Default ris International Journal of Business Marketing and Management (IJBMM) Page 97 βˆ— 𝑙𝑛| 𝑙1 βˆ’ 𝑙2 𝑙1 βˆ’ 𝑙2𝑒0.5𝜎2(𝑙1βˆ’π‘™2)(1βˆ’πœŒ2)(Ξ³+v1)(π‘‡βˆ’π‘‘) |),ρ β‰  Β±1 Ξ±2 π‘˜πœ” 2(π‘˜ + Ξ±Ο‚)(Ξ³ + v1) (T βˆ’ t) + 1 2 ( Ξ± (k + Ξ±Ο‚)(Ξ³ + v1) )2 π‘˜πœ”(1 βˆ’ 𝑒(π‘˜+Ξ±Ο‚)(π‘‡βˆ’π‘‘) ),ρ = 1 Ξ±2 π‘˜πœ” 2(π‘˜ βˆ’ Ξ±Ο‚)(Ξ³ + v1) (T βˆ’ t) + 1 2 ( Ξ± (k βˆ’ Ξ±Ο‚)(Ξ³ + v1) )2 π‘˜πœ”(1 βˆ’ 𝑒(π‘˜βˆ’Ξ±Ο‚)(π‘‡βˆ’π‘‘) ),ρ = βˆ’1, k β‰  Ξ±Ο‚ π‘˜πœ” ,(Tβˆ’t)Ξ±-2 4(Ξ³+v1) ,ρ = βˆ’1,k = Ξ±Ο‚. End. 1.6 Optimal reinsurance and investment decisions before default This section considers the optimal pre-default reinsurance-investment strategy and the value function expression based on the previous section, when H(t)=0, 0 ≀ t ≀ Ο„ ∧ T.Let the solution of the default prior value function have the following form: V(T, x, l, 0) = βˆ’ 1 𝛾 𝑒π‘₯𝑝{βˆ’π›Ύπ‘’π‘Ÿ(π‘‡βˆ’π‘‘)(𝑑)π‘₯ + 𝐾(𝑑, 𝑙)}#(22) Satisfying the boundary conditions V(T, x, l, 0)=V(x), K(T, l)=0, the HJB equation is transformed into: 𝑉𝑑 + ,πœ‹(𝛼𝑙 βˆ’ Ξ¦1βˆšπ‘™) + Ξ²Ξ΄(1 βˆ’ Ξ”) + λμX(ΞΈ1 + Ξ·1π‘ž1(𝑑) βˆ’ 1)) +Ξ»pΞΌY .ΞΈ2 + Ξ·2(π‘ž2(𝑑) βˆ’ 1)√(π‘ž1𝛾1)2 + (π‘ž2𝛾2)2 + 2ρ Μ‚π‘ž1π‘ž2𝛾1𝛾2Ξ¦01 𝑉 π‘₯ + 1 2 ((πœ‹)2 𝑙 + Ξ»(π‘ž1πœπ‘‹)2 + Ξ»p(π‘ž2πœπ‘Œ)2 + 2Ξ»pπœπ‘‹πœπ‘Œ)𝑉 π‘₯π‘₯ +πœ‹π‘™πœΟπ‘‰π‘₯𝑙 + (k(Ο‰ βˆ’ l) βˆ’ Ξ¦1πœπœŒβˆšπ‘™ βˆ’ Ξ¦2𝜍√1 βˆ’ 𝜌2βˆšπ‘™))Vl + 1 2 Ο‚2 lVll +𝑉(𝑒𝛾β(t)΢𝐻1(𝑑)+𝐾(𝑑,𝑙)βˆ’G(𝑑,𝑙) βˆ’ 1)𝑕𝑝 βˆ’ Ξ¦0 2 2v0 𝛾 βˆ’ Ξ¦1 2 2v1 𝛾 βˆ’ Ξ¦2 2 2v2 𝛾 βˆ’ (Ξ¦3 ln Ξ¦3 βˆ’ Ξ¦3 + 1)hp (1 βˆ’ h) 2v2 𝛾 = 0 Similarly find the partial derivative of V: { 𝑉𝑑 = [Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘) + Kt’]V, 𝑉 π‘₯ = βˆ’Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘) V 𝑉 π‘₯π‘₯ = Ξ³2 𝑒2π‘Ÿ(π‘‡βˆ’π‘‘) π‘‰οΌŒπ‘‰π‘₯𝑙 = βˆ’Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘) 𝐾𝑙𝑉 𝑉𝑙 = 𝐾𝑙𝑉, 𝑉𝑙𝑙 = 𝐾𝑙𝑙𝑉 + 𝐾𝑙 2 𝑉 Bringing the above expression into the HJB equation and fixing the reinsurance-investment strategy, the minimum point of Ξ¦ is obtained according to the first-order condition as: { Ξ¦0 βˆ— = v0π‘’π‘Ÿ(π‘‡βˆ’π‘‘) √(π‘ž1𝛾1)2 + (π‘ž2𝛾2)2 + 2ρ Μ‚π‘ž1π‘ž2𝛾1𝛾2 Ξ¦1 βˆ— = v1π‘’π‘Ÿ(π‘‡βˆ’π‘‘) πœ‹βˆšπ‘™, Ξ¦2 βˆ— = βˆ’πœπœŒβˆšπ‘™K𝑙v2 1 𝛾 Ξ¦3 βˆ— = 𝑒π‘₯𝑝* v3 𝛾 .π‘’βˆ’π›ΎΞ²(t)ΞΆπ‘’π‘Ÿ(π‘‡βˆ’π‘‘)+𝐺1(𝑑,𝑙)βˆ’G(𝑑,𝑙) βˆ’ 1/+ #(23) After bringing (23) into the HJB equation, by inverting the investment strategy we can obtain: Ο€βˆ— = Ξ± βˆ’ ςρ(Ξ³ + v1)Kl (Ξ³ + v1)π‘’π‘Ÿ(π‘‡βˆ’π‘‘) , Ξ²βˆ— = ln 1 βˆ†Ξ¦3 + K(t, l) βˆ’ G(t, l) ΞΆΞ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘) and obviously the expressions for the reinsurance strategy before and after the bond default are the same, so that 𝑔1(𝑑, q1, q2) = 𝑓1(𝑑, q1, q2) = βˆ’,ΞΌXΞ·1q1 + pΞΌYΞ·2q2- + 1 2 ,(q1Ο‚X)2 + p(q2Ο‚Y)2 + pΟ‚XΟ‚YΞΌXΞΌYΞ³2 -(Ξ³ + v0)π‘’π‘Ÿ(π‘‡βˆ’π‘‘) Bringing Ξ΅βˆ— = (Ο€βˆ— , Ξ²βˆ— , π‘ž1 βˆ— , π‘ž2 βˆ— ) into the equation, after finishing, we get: ,βˆ’Ξ³π‘Ÿπ‘’π‘Ÿ(π‘‡βˆ’π‘‘) + Kt’- βˆ’ ,λμX(ΞΈ1 βˆ’ Ξ·1) +Ξ»pΞΌY(ΞΈ2 βˆ’ Ξ·2)-Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘) + k(Ο‰ βˆ’ l)Kl + 1 2 Ο‚2 l(Kll + Kl 2 ) βˆ’Ξ»Ξ³er(Tβˆ’t) 𝑔1(q1 βˆ— , q2 βˆ— , t) βˆ’ g2(Ο€βˆ— , t) βˆ’ g3(Ξ²βˆ— , t) = 0#(24) Where
  • 8. Robust Optimal Reinsurance and Investment Problem with p-Thinning Dependent and Default ris International Journal of Business Marketing and Management (IJBMM) Page 98 𝑔2(Ο€βˆ— , t) = Ο€βˆ— π‘™π‘’π‘Ÿ(π‘‡βˆ’π‘‘)(ςρ𝐾𝑙(Ξ³ + v1) βˆ’ 𝛼) + 1 2 (Ξ³ + v1)lΟ€βˆ—2 𝑒2π‘Ÿ(π‘‡βˆ’π‘‘) (25) 𝑔3(Ξ²βˆ— , t) = βˆ’ (Ξ¦3 βˆ’ 1)Ξ³hp v3 βˆ’ δβγer(Tβˆ’t) #(26) The derivative of equation (26) with respect to Ξ² : Ξ²βˆ— = ln 1 βˆ†Ξ¦3 + K(t, l) βˆ’ G(t, l) ΞΆΞ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘) #(27) Bringing it into Ξ¦3 βˆ— get : hp v3 Ξ¦3(𝑑) ln Ξ¦3(𝑑) + hp Ξ¦3(𝑑) βˆ’ 𝛿 𝜁 = 0. The equation has dimension one positive roots Ξ¦3. Let K(t, l)=K1(t)l + K2(t),satisfy the boundary conditions K1(T)=0,K2(T) = 0,bring it to HJB equation(24), eliminating the effect of l on the equation yields two equations: K1 ’ βˆ’ k𝐾1 + 1 2 Ο‚2 𝐾1 2 βˆ’ 𝐾1 βˆ’ 𝐺1 ΞΆ Ξ΄ βˆ’ (ςρ𝐾1 + Ξ±(Ξ΄ βˆ’ r))Ξ±(Ξ΄ βˆ’ r) + 1 2 (ςρ𝐾1 + Ξ±(Ξ΄ βˆ’ r)) 2 βˆ’ (ςρ𝐾1 + Ξ±(Ξ΄ βˆ’ r))ςρ𝐾1 = 0#(28) K2 ’ βˆ’ ,λμX(ΞΈ1 βˆ’ Ξ·1) + Ξ»pΞΌY(ΞΈ2 βˆ’ Ξ·2)-Ξ³π‘’π‘Ÿ(π‘‡βˆ’π‘‘) + Ξ»pΟ‚XΟ‚YΞ³2 (π‘’π‘Ÿ(π‘‡βˆ’π‘‘) )2 +kω𝐾1 βˆ’ ln 1 βˆ†Ξ¦3 + 𝐾2 βˆ’ 𝐺2 ΞΆ Ξ΄ + (1 βˆ’ 1 βˆ†Ξ¦3 ) hp βˆ’ f(t, π‘ž1 βˆ— , π‘ž2 βˆ—) = 0#(29) When ρ β‰  Β±1,equation (28) is the first-order RICCATI equation that,from the existence uniqueness of the solution 𝐾1 = 𝐺1.Then we solve equation(29):let I(t)=𝐾2 βˆ’ 𝐺2, Satisfying the boundary condition I(T) = 0, then using (29) and (21) we get:I’ = K2 ’ βˆ’ G2 ’ = Ξ΄ ΞΆ I + Ξ΄ ΞΆ ln 1 βˆ†Ξ¦3 + Ξ³(Ξ¦3βˆ’1) v3 hp , I = (ln 1 βˆ†Ξ¦3 + βˆ† βˆ’ 1)𝑒 βˆ’ Ξ΄ ΞΆ (Tβˆ’t) βˆ’ ln 1 βˆ†Ξ¦3 βˆ’ βˆ† + 1.Thus : 𝐾2 = (ln 1 βˆ†Ξ¦3 + Ξ³βˆ†(Ξ¦3 βˆ’ 1) v3 ) 𝑒 βˆ’ Ξ΄ ΞΆ (Tβˆ’t) βˆ’ ln 1 βˆ†Ξ¦3 + Ξ³βˆ†(Ξ¦3 βˆ’ 1) v3 + 𝐺2(30) Theorem III.3 The optimal pre-default investment and bond investment strategies are: Ο€βˆ— = Ξ± βˆ’ ςρ(Ξ³ + v1)Kl (Ξ³ + v1)π‘’π‘Ÿ(π‘‡βˆ’π‘‘) , π›½βˆ— = 𝑣3ln 1 ΔΦ3 + Δ𝛾(Ξ¦3 βˆ’ 1)(𝑒 βˆ’ 𝛿 𝜁 (Tβˆ’t) βˆ’ 1) 𝑣3πœπ›Ύπ‘’π‘Ÿ(π‘‡βˆ’π‘‘) The expression of the value function before default is: 𝑉(𝑑, π‘₯, 𝑙, 0) = βˆ’ 1 𝛾 exp{βˆ’π›Ύπ‘’π‘Ÿ(π‘‡βˆ’π‘‘) π‘₯ + K1(𝑑)𝑙+K2(𝑑)+ Where K1(t) = G1(t),as the formula (14)-(17),K2(t) as the formula (30). IV. Parameter Sensitivity analysis In this chapter, we give several numerical examples to test the effect of model parameters on the optimal strategy. First, to make the parameters more realistic, the model parameters for credit bonds are set as follows, based on the estimates from Berndt (2008)[22] and Collin-Dufrensn & Solnik (2001)[23], which are estimated from market data:1/Ξ” = 2.53, hQ = 0.013, 𝜁 = 0.52.The claim amounts for the first and second categories of insurance respectively meet the parameters of λ𝑋 = 1.5, Ξ»π‘Œ = 1exponential distribution . If no special statement is made, other model parameters are assumed as follows:: t = 3, T = 10, π‘Ÿ = 0.06, 𝛾 = 4,η1 = 2, Ξ·2 = 3, p = 0.5, 𝜈0 = 𝜈1 = 𝜈3 = 1, ρ = 1, Ο‚ = 0.16, π‘˜ = 2, Ξ± = 1.5. 1.7 Influence of parameters on optimal reinsurance strategy Figure.1-2 represents the variation of the optimal reinsurance strategy with the parameters. where Figure.1 shows the variation of reinsurance strategy with probability p. When p increases, the insurer will reduce the amount of reinsurance retention for class I reinsurance and increase the amount of reinsurance retention for class II reinsurance. figure.2 represents the effect of risk aversion coefficient 𝛾 on reinsurance strategy, when the risk aversion coefficient is larger, the insurer will reduce the reinsurance strategy and purchase more reinsurance to diversify Claims risk.
  • 9. Robust Optimal Reinsurance and Investment Problem with p-Thinning Dependent and Default ris International Journal of Business Marketing and Management (IJBMM) Page 99 Figure. 1 Figure. 2 1.8 Influence of parameters on optimal investment strategy Where Figure.3 represents the effect of the risk-free rate r on the optimal investment strategyΟ€βˆ— , when the risk- free rate increases, the insurer will invest less in risky assets and more assets in risk-free assets. Figure.4 represents the effect of the volatile reversion rate k on the optimal investment strategyΟ€βˆ— . When the reversion rate increases, the insurer will increase its investment in risky assets. Figure. 3 Figure. 4 Figure.5 shows the impact of risk premium 1 Ξ” on credit bonds, when the risk premium increases, investment in credit bonds is increased. Figure.6 indicates that when the default loss rate 𝜁 increases, insurers will invest less in credit bond. Figure. 5 Figure. 6 1.9 Influence of ambiguity aversion coefficient on strategy Figure.7 to Figure.9 represent the effect of optimal reinsurance-investment strategy subject to ambiguity aversion sparsity. It can be seen that as the ambiguity aversion coefficient increases, the insurer's uncertainty about the model increases and therefore reduces its optimal reinsurance-investment strategy. 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 p 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 q1 q2 2 2.5 3 3.5 4 4.5 5 5.5 6 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 q1 q2 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 r 0.05 0.1 0.15 0.2 0.25 0.3 1 1.5 2 2.5 3 3.5 4 4.5 5 k 0.222 0.2225 0.223 0.2235 0.224 0.2245 0.225 0.2255 0.226 1 2 3 4 5 6 7 8 1/ 0 0.5 1 1.5 2 2.5 3 3.5 4 1 , =1.5 2 , =2.53 3 , =5 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.5 1 1.5 2 2.5 3 3.5 1 ,1/ =1.5 2 ,1/ =2.53 3 ,1/ =5
  • 10. Robust Optimal Reinsurance and Investment Problem with p-Thinning Dependent and Default ris International Journal of Business Marketing and Management (IJBMM) Page 100 Figure. 7 Figure. 8 Figure. 9 V. Conclusion This paper assumes that the insurer owns two types of insurance business with sparse dependence risk, and the claim process is described by a diffusion approximation model, and secondly, the insurer expands its investment types by investing in the financial market with a stock, a risk-free asset and a credit bond, with the stock price described by a Heston model and the credit bond price described by an approximate model. The credit bond price is described by the approximate model, and considering that the insurer is ambiguous averse, the robust optimal reinsurance-investment problem is established, and the explicit expressions of the robust optimal reinsurance-investment and optimal value function are obtained by using stochastic control theory, dynamic programming principle, and HJB equation, and sensitivity analysis is performed on the model parameters. Based on this paper, further discussions can be made: 1) other situations of the claim process can be considered: such as jump diffusion or common shock. 2) time lag effects can be considered. 3) game problems can be considered. References [1]. Browne S . Optimal investment policies for a firm with a random risk process: Exponential utility and minimizing the probability of ruin[J]. Mathematics of Operations Research, 1995, 20(4):937-958. [2]. Zhang Y , Zhao P , Teng X , et al. Optimal reinsurance and investment strategies for an insurer and a reinsurer under Hestons SV model: HARA utility and Legendre transform[J]. Journal of Industrial and Management Optimization, 2017, 13(5). [3]. Zhu S , Shi J . Optimal Reinsurance and Investment Strategies Under Mean-Variance Criteria: Partial and Full Information[J]. Journal of Systems Science and Complexity: English Edition, 2022, 35(4):22. [4]. LI, Danping, RONG, et al. Optimal Investment Problem for an Insurer and a Reinsurer[J]. Journal of Systems Science and Complexity: English Edition.,2015(6):18. [5]. A D L , B X R A , A H Z . Time-consistent reinsurance–investment strategy for an insurer and a reinsurer with mean–variance criterion under the CEV model[J]. Journal of Computational and Applied Mathematics, 2015, 283:142-162. [6]. Chun-Xiang A , Gu A L , Shao Y . Optimal Reinsurance and Investment Strategy with Delay in Heston's SV Model[J]. Journal of the Chinese Society of Operations Research (English), 2021, 9(2):27. [7]. Xiaoqin Gong, Shixia Ma, Qing Huang,, Robust optimal investment strategies for insurance and reinsurance companies under stochastic interest rates and stochastic volatility (in English) [J]. Journal of Nankai University: Natural Science Edition, 2019, 52(6):11. [8]. Grandell J . Aspects of Risk Theory[M]. World Publishing Co. 1991. [9]. Zhao H , Rong X , Zhao Y . Optimal excess-of-loss reinsurance and investment problem for an insurer with jump-diffusion risk process under the Heston model[J]. INSURANCE -AMSTERDAM-, 2013. [10]. Ceci C , Colaneri K , Cretarola A . Optimal reinsurance and investment under common shock dependence between financial and actuarial markets[J]. Insurance: Mathematics and Economics, 2022, 105. [11]. Zhang P . Optimal excess-of-loss reinsurance and investment problem with thinning dependent risks under Heston model[J]. Journal of Computational and Applied Mathematics, 2021, 382(1). [12]. Li S , Qiu Z . Optimal Time-Consistent Investment and Reinsurance Strategies with Default Risk and Delay under Heston's SV Model[J]. Mathematical Problems in Engineering, 2021, 2021(1):1-36. [13]. Zhu G . Time-consistent non-zero-sum stochastic differential reinsurance and investment game under default and volatility risks[J]. Journal of Computational and Applied Mathematics, 2020, 374. [14]. Zhenlong Chen, Weijie Yuan, Dengfeng Xia. Optimal Reinsurance-Investment Strategy with Default Risk Based on Heston's SV Model [J]. Business Economics and Management, 2021, 000(005):56-70. 0 1 2 3 4 5 6 7 8 9 10 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 q1 q2 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 1 0.1 0.15 0.2 0.25 1 1.5 2 2.5 3 3.5 4 3 0.3442 0.34425 0.3443 0.34435 0.3444 0.34445 0.3445 0.34455 0.3446 1
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