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lecture09

# lecture09 - Induction COMPUTER SCIENCE 51 Spring 2009...

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3/3/2009 1 COMPUTER SCIENCE 51 Spring 2009 cs51.seas.harvard.edu Prof. Greg Morrisett Prof. Ramin Zabih Induction Computers are discrete & finite But we don’t want to argue based on their current hardware Want our programs to run correctly and efficiently on arbitrary inputs Prove an infinite set of statements? P[1] = “my factorial program is correct when the input is 1” P[2], P[3], … Gets old fast! Induction recipe 1. What variable n are you doing induction on, and what is its type? 2. What statement P[n] will you prove? 3. Prove P[1] (first element, can be 0) 4. For an arbitrary n, prove P[n+1] follows from P[n]. Induction Hypothesis (must be used!) Recipe use (math) 1. Variable is positive integer n 2. Statement: 3. Proof of P[1] 4. Want to show (via IH!): English ambiguity in IH Need: for all n, P[n] implies P[n+1] We pick an n and assume P[n] (IH) Then derive P[n+1] It’s always true that For all n, P[n] implies For all n, P[n+1] Math notation helps n ( P[n] P[n+1] ) versus n ( P[n]) n(P[n+1] ) Example notes IH versus statement This theorem is very easy in binary Related to many exponentially growing functions “Everything is at the leaves”

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