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# Proofs

Example 2. Prove the following results:

(a) If c is a non-zero rational number , then c π is irrational.
(b) If m3 is even, then m is even.
(c) For all real numbers x and y , if x + y > 0, then x > 0 or y > 0.
(d) Let n be an integer. If n2 is divisible by 3, then n is divisible by 3.
(e) Let x be a real number . If for all , then x = 0 .

(a) Proof. (By contradiction) Let c be a non- zero rational number, and suppose that c π
is rational. Then c π = m / n , where m , n are integers and n ≠ 0. Moreover, c = j / k ,
where j , k are integers and k ≠ 0, but also j ≠ 0 because c is non-zero. We then have The products of integers k m and jn are still integers, and jn cannot be 0 because
neither n nor j is 0. Thus, π is in the form of a rational number, which is a
contradiction because π is irrational. Ergo, c π must be irrational. QED
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(b) Claim: If m3 is even, then m is even. We shall prove the contrapositive instead; we
shall assume that m is not even and show that m3 is not even. That is, we shall assume
that m is odd and show that m3 is odd.

Proof. Assume that m is odd. Then m = 2 k +1 for some integer k . Then,

m3
= (2 k + 1)3
= 8k3 +12k2 + 6k + 1
= 2(4k3 + 6k 2 + 3k) + 1
= 2l + 1,

where l is the integer 4k3 + 6k2 + 3k . Thus, m3 is odd because it is written in the form
of an odd integer. By contrapositive, if m3 is even, then m is even. QED
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(c) Proof. Let x and y be real numbers . Assume that x ≤ 0 and y ≤ 0. Then by adding ,
we have x + y ≤ 0 + 0 = 0. That is, x + y ≤ 0. Hence, x ≤ 0 and y ≤ 0 implies that
x + y ≤ 0. By contrapositive, if x + y > 0, then x > 0 or y > 0. QED

(d) Claim: For an integer n , if n2 is divisible by 3, then n is divisible by 3.

Proof. Assume n is not divisible by 3. We then can apply the Fundamental Theorem of
Arithmetic to write n uniquely as a product of powers of primes as follows: where ri are natural numbers and the pi are distinct primes. Because n is not divisible
by 3, then pi ≠ 3 for 1 ≤ i ≤ k . By squaring we obtain which is a prime factorization of n 2. By uniqueness of prime factorization, the above pi
for 1 ≤ i ≤ k are the only prime factors of n 2. Because none of the pi equals 3, then n2 is
not divisible by 3. By contrapositive, if n2 is divisible by 3, then n must also be divisible
by 3. QED
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Note: The preceding result can be generalized as follows:
Let p be prime and suppose k ≥ 2 . If nk is divisible by p , then n is divisible by p .
(In the proof of (d), simply replace 3 with p and replace the exponent 2 with k .)
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(e) Claim: If all , then x = 0 .

Proof. Suppose all We must show that x = 0 . So suppose that x ≠ 0 .
Then x < 0 or x > 0. In either case, we have . Now let . For this , we then have , which contradicts the assumption. Thus we must
have that x = 0 . QED

Note: We also could have argued by contrapositive: Assume x ≠ 0 . Then we showed
that there exists an for which fails. So if x ≠ 0 , then it is not true that for all . By contrapositve, if it is true that for all , then x = 0 .

Double Implication

A double implication is of the form p ↔ q, which is read “ p if and only if q ”. This
statement means p → q and q → p and both implications must be proven.

Example 3. Let n be an integer. Then n is even if and only if n2 is even.

Proof. Suppose first that n is even. Then n = 2k , for some integer k . So then

n2 = (2 k )2 = 4k2 = 2(2k2 ).

Because 2k2 is also an integer, we see that n2 must be even.

Next we must prove that if n2 is even, then n is even. But we shall prove the
contrapositive instead. So assume that n is not even, i.e., that n is odd. Then n = 2k +1
for some integer k . We then have

n2 = (2k +1)2 = 4k 2 + 4k + 1 = 2(2k2 + 2k) +1 = 2 j +1,

where j = 2k2 + 2k is an integer. Hence, n2 is odd; that is, n2 is not even. We have
proven that if n is not even, then n2 is not even. By contrapositive, if n2 is even, then n
is even. From both directions, we now have that n is even if and only if n2 is even.
QED

Note: The second direction also follows from the generalization of Part (d) using the
prime p = 2 . In other words, if 2 divides n2 then 2 divides n .
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Theorem. is irrational.

Proof. Assume is rational. Then we can write = m / n , where m and n are
integers with n ≠ 0. Furthermore, we can assume that the fraction is reduced so that m
and n have no common divisors . Then by squaring the fraction , we have or 2n2 = m2 .

Thus, m2 is even because it is divisible by 2. It follows that the integer m must be
even (by Example 3). So we may write m = 2k for some integer k . Then

2n2 = m2 = (2 k)2 = 4k 2 = 2(2 k2 ), which gives n2 = 2k 2.

Thus, n2 is even because it is divisible by 2. It follows that the integer n also must
be even. So both m and n are even and therefore both are divisible by 2. But this fact
contradicts the assumption that we have chosen m and n to have no common divisors.
This contradiction leads us to conclude that cannot be written as a fraction. Thus, is irrational.  QED

Corollary. For any prime p , is irrational.

Exercises

Prove the following results in a formal, elegantly written, mathematical style :

(a) For all integers m and n , if m is even and n is odd, then m + n is odd.

(b) Let a be an integer. If a divides b , then b is an integer and a divides b2.

(c) If x and y are rational numbers, then x + y is a rational number.

(d) Let m and n be integers. If m n is even, then m is even or n is even.

(e) Let m and n be integers. If m + n is odd, then m is odd or n is odd but not both.

(f) Let x and y be real numbers. Then x = y if and only if for every .

(g) For any prime p , is irrational.

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