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**PROOF BY CONTRADICTION**

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**proof by contradiction**

Let r be a proposition. A proof of r by contradiction consists of proving that not(r) implies a contradiction, thus concluding that not(r) is false, which implies that r is true.

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**proof by contradiction**

In particular if r is if p then q then not(r) is logically equivalent to p AND (not(q)). This can be verified by constructing a truth table.

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**proof by contradiction**

So a true proposition if p then q may be proved by contradiction as follows: Assume that p is true and q is false, and show that this assumption implies a contradiction.

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**proof by contradiction**

One way of proving that the assumption that p is true and q is false implies a contradiction is proving that this assumption implies that p is false. Since the same assumption also implies that p is true, we conclude that the assumption implies that p is true and p is false, which is a contradiction.

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**proof by contradiction**

EXAMPLE: Prove that the sum of an even integer and a non-even integer is non-even. (Note: a non-even integer is an integer that is not even.)

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**proof by contradiction**

We have to prove that for every even integer a and every non-even integer b, a+b is non-even. This is the same as proving that For all integers a,b, if [a is even and b is non-even] then [a+b is non-even].

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**proof by contradiction**

We prove that by contradiction. Assume that [a is even and b is non-even], and that [a+b is even]. So for some integers m,n, a=2m and a+b=2n. Since b=(a+b)-a, b=2n-2m=2(n-m). We conclude that b is even. This leads to a contradiction, since we assumed that b is non-even.

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CS5371 Theory of Computation

CS5371 Theory of Computation

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