## Contrapositive Proofs

The following is the list of problems for Chapter 5 of the Book of Proof (page 110). There is a forum open at the end, so you can ask questions. It is a great way to interact with the instructor and with other students in your class, should you need some assistance with any question. Please, do not post solutions.

A. Use the method of contrapositive proof to prove the following statements. (In each case you should also think about how a direct proof would work. You will find in most cases that contrapositive is easier.)

1. Suppose $$n \in \mathbb{Z}$$. if $$n^2$$ is even, then $$n$$ is even.
2. Suppose $$n \in \mathbb{Z}$$. if $$n^2$$ is odd, then $$n$$ is odd.
3. Suppose $$a, b \in \mathbb{Z}$$. If $$a^2(b^2-2b)$$ is odd, then $$a$$ and $$b$$ are odd.
4. Suppose $$a, b, c \in \mathbb{Z}$$. If $$a$$ does not divide $$bc$$, then $$a$$ does not divide $$b$$.
5. Suppose $$x \in \mathbb{R}$$. If $$x^2+5x<0$$, then $$x < 0$$.
6. Suppose $$x \in \mathbb{R}$$. If $$x^3-x>0$$ then $$x > -1$$.
7. Suppose $$a,b \in \mathbb{Z}$$. If both $$ab$$ and $$a+b$$ are even, then both $$a$$ and $$b$$ are even.
8. Suppose $$x \in \mathbb{R}$$. If $$x^5-4x^4+3x^3-x^2+3x-4 \geq 0$$, then $$x \geq 0$$.
9. Suppose $$n \in \mathbb{Z}$$. If $$3 \!\nmid\! n^2$$, then $$3 \!\nmid\! n$$.
10. Suppose $$x, y, z \in \mathbb{Z}$$ and $$x \neq 0$$. If $$x \!\nmid\! yz$$, then $$x \!\nmid\! y$$ and $$x \!\nmid\! z$$.
11. Suppose $$x, y \in \mathbb{Z}$$. If $$x^2(y+3)$$ is even, then $$x$$ is even or $$y$$ is odd.
12. Suppose $$a \in \mathbb{Z}$$. If $$a^2$$ is not divisible by 4, then $$a$$ is odd.
13. Suppose $$x \in \mathbb{R}$$. If $$x^5 + 7x^3 + 5x \geq x^4 + x^2 + 8$$, then $$x \geq 0$$.

B. Prove the following statements using either direct or contrapositive proof. Sometimes one approach will be much easier than the other.

1. If $$a, b \in \mathbb{Z}$$ and $$a$$ and $$b$$ have the same parity, then $$3a +7$$ and $$7b-4$$ do not.
2. Suppose $$x \in \mathbb{Z}$$. If $$x^3-1$$ is even, then $$x$$ is odd.
3. Suppose $$x \in \mathbb{Z}$$. If $$x+y$$ is even, then $$x$$ and $$y$$ have the same parity.
4. If $$n$$ is odd, then $$8 \vert (n^2-1)$$.
5. For any $$a, b \in \mathbb{Z}$$, it follows that $$(a+b)^3 \equiv a^3 + b^3 \pmod{3}.$$
6. Let $$a, b \in \mathbb{Z}$$ and $$n \in \mathbb{N}.$$ If $$a \equiv b \pmod{n}$$ and $$a \equiv c \pmod{n} n,$$ then $$c \equiv b \pmod{n}.$$
7. If $$a \in \mathbb{Z}$$ and $$a \equiv 1 \pmod{5},$$ then $$a^2 \equiv 1 \pmod{5}.$$
8. Let $$a, b \in \mathbb{Z}$$ and $$n \in \mathbb{N}.$$ If $$a \equiv b \pmod{n},$$ then $$a^3 \equiv b^3 \pmod{n}.$$
9. Let $$a \in \mathbb{Z}, n \in \mathbb{N}.$$ If $$a$$ has a remainder $$r$$ when divided by $$n,$$ then $$a \equiv r \pmod{n}.$$
10. Let $$a, b, c \in \mathbb{Z}$$ and $$n \in \mathbb{N}.$$ If $$a \equiv b \pmod{n},$$ then $$ca \equiv cb \pmod{n}.$$
11. If $$a \equiv b \pmod{n}$$ and $$c \equiv d \pmod{n},$$ then $$ac \equiv db \pmod{n}.$$
12. If $$n \in \mathbb{N}$$ and $$2^n - 1$$ is prime, then $$n$$ is prime.
13. If $$n = 2^k -1$$ for $$k \in \mathbb{N},$$ then every entry in Row $$n$$ of Pascal’s Triangle is odd.
14. If $$a \equiv 0 \pmod{4}$$ or $$a \equiv 1 \pmod{4},$$ then $$\binom{a}{2}$$ is even.
15. If $$n \in \mathbb{Z},$$ then $$4 \!\nmid\! (n^2-3).$$
16. If integers $$a$$ and $$b$$ are not both zero, then $$\gcd(a,b) = \gcd(a-b,b).$$
17. If $$a \equiv b \pmod{n},$$ then $$\gcd(a,n) = \gcd(b,n).$$
18. Suppose the division algorithm applied to $$a$$ and $$b$$ yields $$a = qb+r.$$ Then $$\gcd(a,b) = \gcd(r,b).$$