# CMPSCI 250 Discussion #1: What Is A Proof?

#### 12 Sepember 2006

I had intended to talk about preconditions, postconditions, and loop invariants in the discussion meeting but abandoned the example I was doing in order to give you more time to work on the other questions. So you didn't have much background for the last two questions, and hardly any of you got to them in any case. Have a look at Excursion 1.3 in the textbook to see what I was talking about.

1. Prove that if any natural is even, then its successor is odd, and vice versa.

Since x is even, it is equal to 2y for some natural y. Its successor is then equal to 2y+1, which is twice a natural plus 1 and thus is odd.

Now assume x is odd, so that x = 2y + 1 for some natural y. The successor of x is 2y + 2 = 2(y + 1), which is even because it is twice a natural.

2. Prove that if any positive natural is even, then its predecessor is odd, and vice versa.

First assume that x is even and positive. Since it is even, it equals 2y for some y. If y were 0, then 0 would be 0, which it isn't, so we can assume that y has a predecessor y - 1. By arithmetic, x's predecessor x - 1 i equal to 2y - 1 = 2(y-1) + 1. So this predecessor is twice a natural plus one and is thus odd.

Now assume that x is odd, so that x = 2y + 1 for some y. The predecessor of x is 2y, which is twice a natural and thus is even.

3. Prove that every natural is either odd or even. (Hint: By the Least Number Axiom, if any natural is neither odd nor even, there's a least such natural. Is it 0? Show that the existence of a least such natural leads to a contradiction.)

Assume that there is some natural that is neither odd nor even, and let x be the least such natural. This x cannot be 0, because 0 = 2(0) is even. So x has a predecessor x-1, and this number must be either odd or even because x is the least number that is neither and x-1 is less than x. But by (1), if x-1 is even then x is odd and if x-1 is odd then x is even. So either way, x is either even or odd, and we have a contradiction. Since the assumption that there was a neither-odd-nor-even natural led to a contradiction, no such natural exists.

4. Prove that no natural is both odd and even. (Similar to (3) -- get a contradiction by assuming that some natural is both.) Assume some both-even-and-odd natural exists, and by the Least Number Axiom let x be the least such natural. This x cannot be 0 because 0 is not odd -- if it were equal to 2z + 1 then 2z would be the predecessor of 0, which does not exist. So x must have a predecessor x-1. By (2), x-1 is even because x is odd, and x-1 is also odd because x is even. But then x-1 is both even and odd and is less than x, contradicting the fact that x is the least natural that is both even and odd. Since the existence of a both-even-and-odd natural leads to a contradiction, no such natural exists.
5. Prove that the following pseudo-Java method is partially correct and that it terminates given any natural as its argument:

``````     public boolean isEven (natural x)
{// returns whether x is an even number
natural i = 0;
while (true) {
if (x == 2 * i) return true;
if (x == 2 * i + 1) return false;
i++;}}
``````

For partial correctness we must prove that if the method terminates, it gives the correct answer. The only way it can terminate is through one of the return statements. If we return "true", we have just seen that x is 2 times a natural, so x is even. If we return "true", we have just seen that x is 2 times some natural plus 1, so we know that x is odd, and then by (4) we know it is not even.

For termination on any given x, we need to know that the loop will find the correct i such that x = 2i or x = 2i + 1 is true. (Since x is either even or odd by (3), such an i must exist.) Assume that there is a number for which the method does not terminate, and let z be the least such number. Let k be the natural such that z = 2k or z = 2k+1. This z cannot be 0 or 1, because clearly the code terminates on 0 or 1 with i = 0. So the natural z - 2 exists, and the method terminates on input z - 2. Since z - 2 is smaller than z, the method terminates on input z - 2. Since z - 2 is equal to either 2(k - 1) or 2(k - 1) + 1, the method must eventually set i to be k - 1. But then, on the next time through the loop, it sets i to k and then one of the `if` statements will cause the method to return.

6. Prove that the following pseudo-Java method is partially correct and that is terminates given any natural as its argument:

``````
public boolean isEven (natural x)
{// returns whether x is an even number
if (x == 0) return true;
if (x == 1) return false;
return !isEven (x - 1);}
``````

To prove partial correctness, assume that there exists a natural on which the method terminates with the wrong answer. Let z be the smallest such natural. This z cannot be 0 (or 1), because we can see that the method gives the right answer on those inputs. So z - 1 must exist, and since z - 1 is smaller than z, if the method terminates on input z - 1 it must give the right answer. But if the method terminates on z, it must do so by calling itself on z - 1 and giving the opposite answer. And by (2), we know that z - 1 is odd if z is even and vice versa, so that `!isEven (z - 1)` correctly tells whether z is even. So the answer on z is correct, contradicting the assumption.

To prove termination, assume that there is a natural on which the method does not terminate and let z be the least such number. This z cannot be 0 or 1 because the method clearly terminates on both of those inputs. So the natural z - 1 exists, and because z - 1 is smaller than z, we know that the method terminates on input z - 1. But then it also terminates on input z, because then it calls itself on input z - 1. This contradicts our assumption.