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Discrete
Mathematics
By
Sadat Sumon

Introduction to Discrete Mathematics
Aug 16, 17, Sep 11 2011
A B
C
a = qb+r gcd(a,b) = gcd(b,r)

What is Discrete Math?
• Discrete mathematics is mathematics that
deals with discrete objects.
• Discrete objects are those which are
separated from (not connected to/distinct
from) each other.

Discrete objects
– Integers, rational numbers (ones that can be
expressed as the quotient of two integers),
automobiles, houses, people etc. are all
discrete objects.
– On the other hand real numbers which include
irrational as well as rational numbers are not
discrete.
• between any two different real numbers there is
another real number different from either of them.
• So they are packed without any gaps and can not
be separated from their immediate neighbors. In
that sense they are not discrete.

Discrete Objects
• In this course we will be concerned with objects
such as integers, propositions, sets, relations
and functions, which are all discrete.
• We are going to learn concepts associated with
them, their properties, and relationships among
them among others.

Why Discrete Math?
• Through DM, we learn formal/theoretical
approaches in computer science.
• Why formal approach is important?
– we can handle infinity or large quantity and
indefiniteness with them
– results from formal approaches are reusable.

Checker
x=0
Start with any configuration with all circles on or below the x-axis.

Checker
x=0
Move: jump through your adjacent neighbour,
but then your neighbour will disappear.

Checker
x=0
Goal: Find an initial configuration with least number of circles to jump up to level k.

K=1
x=0
2 circles to reach level 1.

K=2
x=0
Now we have reduced to the k=1 configuration, but one level higher.

K=3
x=0
This is the configuration for k=2, so jump two level higher.

K=3
x=0

K=4
x=0
Now we have reduced to the k=3 configuration, but one level higher
20 circles (not 16) to reach level 4!

K=5?
a. 39 or below
b. 40-50 circles
c. 51-70 circles
d. 71- 100 circles
e. 101 – 1000 circles
f. 1001 or above
It turns out that it is impossible to move to level 5,
and there is a very interesting mathematical proof of it.

Graph Theory
How to color a map?
How to send data efficiently?
How to schedule exams?

Objectives of This Course
•To learn basic mathematical concepts, e.g. sets, functions, graphs
•To be familiar with formal mathematical reasoning, e.g. logic, proofs
•To improve problem solving skills
•To see the connections between discrete mathematics and computer science

 Introduction to logic
 Motivations
 Basic Definitions
 Logic formula

2 2 2
a b c+ =
Familiar?
Obvious?
cb
a
Pythagorean theorem

cb
a
(i) a c×c square, and then
(ii) an a×a & a b×b square
Good Proof
b-a
We will show that these five pieces can be rearranged into:
b-a
And then we can conclude that

c
cc
a b
c
b-a
Good Proof
The five pieces can be rearranged into:
(i) a c×c square

cb
a b-a
b-a
Good Proof
How to rearrange them into an axa square and a bxb square?

b
a
a ab-a
74 proofs in http://www.cut-the-knot.org/pythagoras/index.shtml
b
Good Proof
b

Mathematical Proof
To prove mathematical theorems, we need a more rigorous system.
The standard procedure for proving mathematical theorems is invented by
Euclid in 300BC. First he started with five axioms (the truth of these
statements are taken for granted). Then he uses logic to deduce the truth
of other statements.
1.It is possible to draw a straight line from any point to any other point.
2.It is possible to produce a finite straight line continuously in a straight line.
3.It is possible to describe a circle with any center and any radius.
4.It is true that all right angles are equal to one another.
5.("Parallel postulate") It is true that, if a straight line falling on two straight lines make the
interior angles on the same side less than two right angles,
the two straight lines, if produced indefinitely, intersect on that side on which are the
angles less than the two right angles.

Statement (Proposition)
A Statement is a sentence that is either True or False
Examples:
Non-examples: x+y>0
x2
+y2
=z2
True
False
2 + 2 = 4
3 x 3 = 8
787009911 is a prime
They are true for some values of x and y
but are false for some other values of x and y.

Logic Operators
F
F
F
T
P Q
FF
TF
FT
TT
QP
AND::=∧
F
T
T
T
P Q
FF
TF
FT
TT
QP
OR::=∨
NOT::=¬ ~p is true if p is false

Compound Statement
p = “it is hot” q = “it is sunny”
It is hot and sunny
It is not hot but sunny
It is neither hot nor sunny

Exclusive-Or
coffee “or” tea  exclusive-or
How to construct a compound statement for exclusive-or?
p q p  q
T T F
T F T
F T T
F F F
Idea 1: Look at the true rowsIdea 1: Look at the true rowsIdea 1: Look at the true rows
Want the formula to be true
exactly when the input belongs
to a “true” row.
The input is the second row exactly if this sub-formula is satisfied
And the formula is true exactly when the input is the second row or the third row.

Exclusive-Or
coffee “or” tea  exclusive-or
How to construct a compound statement for exclusive-or?
p q p  q
T T F
T F T
F T T
F F F
Idea 2: Look at the false rows
Want the formula to be true
exactly when the input does
not belong to a “false” row.
The input is the first row exactly if this sub-formula is satisfied
And the formula is true exactly when the input is not in the 1st
row and the 4th
row.

Logical Equivalence
p q
T T F T F F
T F T T T T
F T T T T T
F F F F T F
Logical equivalence: Two statements have the same truth table
Idea 3: Guess and check
As you see, there are many different ways to write the same logical formula.
One can always use a truth table to check whether two statements are equivalent.

Writing Logical Formula for a Truth Table
Digital logic:
Given a digital circuit, we can construct the truth table.
Now, suppose we are given only the truth table (i.e. the specification),
how can we construct a circuit (i.e. formula) that has the same function?

p q r output
T T T F
T T F T
T F T T
T F F F
F T T T
F T F T
F F T T
F F F F
Use idea 1 or idea 2. Idea 1: Look at the true rows
and take the “or”.
The formula is true exactly when the input is one of the true rows.

Idea 2: Look at the false rows,
negate and take the “and”.
The formula is true exactly when the input is not one of the false row.
p q r output
T T T F
T T F T
T F T T
T F F F
F T T T
F T F T
F F T T
F F F F

DeMorgan’s Laws
Statement: Tom is in the football team and the basketball team.
Negation: Tom is not in the football team or not in the basketball team.
Statement: The number 783477841 is divisible by 7 or 11.
Negation: The number 783477841 is not divisible by 7 and not divisible by 11.
De Morgan’s Law
De Morgan’s Law

DeMorgan’s Laws
T T F F
T F T T
F T T T
F F T T
De Morgan’s Law
De Morgan’s Law

Simplifying Statement
Practice a lot…
DeMorgan
Distributive law

Tautology, Contradiction
A tautology is a statement that is always true.
A contradiction is a statement that is always false. (negation of a tautology)
In general it is “difficult” to tell whether a statement is a contradiction.
It is one of the most important problems in CS – the satisfiability problem.
How this is a negation of the first tautology?

Quick Summary
Key points to know.
1. Write a logical formula from a truth table.
2. Check logical equivalence of two logical formulas.
3. DeMorgan’s rule and other simple logical rules (e.g. distributive).
4. Use simple logical rules to simplify a logical formula.

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Discrete mathematics by sadat sumon

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