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Basic Number Theory

Definitions

• Even and Odd numbers
A number n from Z is called even iff .
A number n from Z is called odd iff .

• Prime and Composite numbers
A number n from Z is called prime iff


A number n from Z is called composite iff
Definitions

• A real number r is called rational if
, such that r = p / q.

Is 0.121212… a rational number?
Every rational number can be written as a repeating or
terminating decimal .
Every integer is a rational number.

• All real numbers which are not rational are called irrational.
• If n and d are both integers, then
n is divisible by d iff .

Notation: d | n


Synonymous statements:
n is a multiple of d
d is a factor of n
d is a divisor of n
d divides n
Theorem: Transitivity of Divisibility

• For all integers a, b and c, if a divides b and b divides c, then
a divides c.

Unique Factorization Theorem for Integers
• Given any integer n>1, there exist a positive integer k, distinct
prime numbers , and positive integers
such that n can be uniquely represented in the standard
factored form: .

(Proof in Section 10.4.)
Quotient and Remainder Theory
• Given any integer n and positive integer d, there exist unique
integers q and r, such that n = dq + r and 0 ≤ r < d. (Proof in Sec 4.4.)

Definitions
Given a nonnegative integer n and a positive integer d,
• n div d: the integer quotient when n is divided by d.
• n mod d: the integer remainder when n is divided by d.

Symbolically, (n div d=q and n mod d =r) ↔ n=dq+r.

Definition
• The parity of an integer refers to the property of an integer to be
even or odd.
Definitions (Floor and Ceiling)
• For any real number x, the floor of x, written , is the unique
integer n such that n ≤ x < n + 1. It is the max of all ints ≤ x.
(Symbolically, )

• For any real number x, the ceiling of x, written , is the
unique integer n such that n – 1 < x ≤ n.
(Symbolically, .)

Examples:
• If k is an integer, what are and ?
• Is the statement   true?
Methods of Proof

Strategies
Direct Proof
Method of Exhaustion
Generalizing from the Generic particular
Indirect Proof
Proof by Contradiction
Proof by Contraposition

Cases
Proving an existential statement
Constructive vs non-constructive proofs of existence
Disproving an existential statement (strategies above)
Proving a universal statement (strategies above)
Disproving a universal statement
Counterexample

Examples

• Show that there is a prime number that can be written as the
sum of two perfect squares.

• Disprove the following statement:
has no non-trivial whole number solutions .
(See a counterexample on P.139.)

• Prove or Disprove the following statements:
For any integers a, b, c: (a | b) → (a | bc).
For any integers a, b, c: .




 

Methods of Proof

Direct Proof:
Express the statement in the form:

Take a particular but arbitrarily chosen x from D so that
P(x) is true.
Show that the conclusion Q(x) is true based on definitions,
previous results , theorems and the rules for logical
inference.

Examples

Prove the following:
• The sum of any two rational numbers is
rational.
• For all integers a, b and c, if a divides b and b
divides c, then a divides c.
• Any integer n>1 is divisible by a prime
number.

Examples

Prove the following:
(proof by cases)

• Any two consecutive integers have opposite
parity.
• The square of any odd integer has the form
8m+1 for some integer m.
 

Examples

Prove the following:
• For all real numbers x and all integers m,

• For any integer is n/2 for even n and (n–
1)/2 for odd n.
• For any nonnegative integer n and any positive
integer d, if and ,
then n = d*q + r and 0 ≤ r < d.

Methods of Proof

Proof by Contraposition:
Express the statement in the form:

Rewrite the statement in the contrapositive form:

Prove the contrapositive by a direct proof:
Take a particular but arbitrarily chosen x from D so that
Q(x) is false.
Show that the conclusion P(x) is false.

Examples

Prove the following:

, n is even ↔ n2 is even.




 

Methods of Proof

Proof by Contradiction:

Suppose that the negation of the statement is true.
Show that this supposition leads logically to a
contradiction
.
Conclude that the statement to be proved is true.


 

Examples

Prove the following:

is irrational.
• There is no integer that is both even and odd.
• The sum of any rational number and any irrational number is
irrational.
• For any integer a and any prime number p,
if p | a, then p does not divide (a + 1).
• The set of prime numbers is infinite.

Common Mistakes

Common mistakes in a proof
• Arguing from example
• Using the same symbol for different variables
• Jumping to a conclusion
• Begging the question
• Misuse of the word if

 
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