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COMPSCI 250: Introduction to Computation Lecture #17: Proof by Induction for Naturals David Mix Barrington 13 October 2017

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Proof by Induction for Naturals Induction as a Proof Rule Example: Sum of First k Odd Numbers is k 2 Common Features of Inductive Proofs Example: 2 n Binary Strings of Length n Example: 2 n Subsets of an n-Element Set Why is Induction Valid? Some Counterintuitive Aspects of Induction
Induction As a Proof Rule Formally, the Law of Mathematical Induction is just a rule that if we have proved certain statements, we are allowed to claim certain additional statements. To use ordinary induction (our topic today), we need a predicate P(x) that has one free variable of type natural . If we prove both “P(0)” and “ x: P(x) P(x+1)”, Then we may conclude “ x: P(x)”.

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Example: Sum of Odd Numbers Let’s look at a simple example. The first odd number is 1 = 2 × 1 - 1, the second is 3 = 2 × 2 - 1, the third 5 = 2 × 3 - 1, and in general the k’th odd number is 2k - 1. (We should actually prove this by induction, but there’s a technicality because we can’t start at 0.) We can see that 1 = 1 2 , 1 + 3 = 2 2 , 1 + 3 + 5 = 3 2 , 1 + 3 + 5 + 7 = 4 2 , and so on. We’ll let P(k) be the statement “the sum of the first k odd numbers is k 2 ”.
Sum of Odd Numbers Proving P(0) is easy -- it says “the sum of the first 0 odd numbers is 0 2 ”, which is true because any empty sum is 0. Now we let x be arbitrary and assume that P(x) is true. So the sum of the first x odd numbers is x 2 . The sum of the first x+1 odd numbers is the sum of the first x, plus the x+1’st odd number which is 2(x+1) - 1 = 2x + 1.

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Sum of Odd Numbers The sum of the first x+1 odd numbers is the sum of the first x, plus the x+1’st odd number which is 2(x+1) - 1 = 2x + 1. So (still assuming that P(x) is true), we get that the sum of the first x+1 odd numbers is x 2 + (2x + 1) = (x+1) 2 . Because we proved P(x) P(x+1) for arbitrary x, we are done.
Features of Inductive Proofs We first proved a base case -- the statement P(0) that we got by substituting 0 for x in the statement P(x). Base cases are usually easy to prove. We then began the inductive step , which is the proof of P(x) P(x+1) for arbitrary x. We assume the truth of P(x), which is called the inductive hypothesis , and use it to prove the inductive goal P(n+1).

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Features of Inductive Proofs Proving the inductive step usually relies on the fact that P(x) and P(x+1) are related statements.
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