# Fibonacci Number Proof

by ryou00730
Tags: fibonacci, number, proof
 P: 29 1. The problem statement, all variables and given/known data The Fibonacci numbers are defined recusively by: F(0) = 0, F(1) = 1, for n > 1, F(n) = F(n − 1) + F(n − 2). Use strong induction to show that F(n) < 2^n for all n. 2. Relevant equations n/a 3. The attempt at a solution I use my base case as F(2) = F(1) + F(0) = 1 which is less than 2^n = 2^2 = 4. After that I am not sure where to go with strong induction...
 Mentor P: 18,346 The induction hypthesis gives us $$F(n)=F(n-1)+F(n-2)\leq 2^{n-1}+2^{n-2}$$ Can you now prove that this is smaller then $$2^n$$?
 P: 29 well 2^n-1 + 2^n-2 is equal to 2^n (2^-1 + 2^-2) = 3/4(2^n) but I don't see the logic behind equating the problem to less than or equal to 2^n-1 + 2^n-2
 Mentor P: 18,346 Fibonacci Number Proof If you want to apply induction? Then what is the induction hypothesis?
 P: 29 Let k>2, for all integers i, with 2<=i
 Mentor P: 18,346 Yes, that is correct. So in particular, we have $$F(n-1)\leq 2^{n-1}~\text{and}~F(n-2)\leq 2^{n-2}$$ So $$F(n-1)+F(n-2)\leq 2^{n-1}+2^{n-2}$$
 P: 29 woops, I meant n>2, so just replace all my k's with n's and so if I know that F(i)<2^i, then F(i-1) + F(i-2) < 2^i-1 + 2^i-2 which is F(i) < 3/4 2^i, so therefore F(i) must also be less than 2^i if its less than 0.75 of it, so the statement is true for all F(n)
 P: 29 does that look about right?
 Mentor P: 18,346 Yeah, that looks fine
 P: 29 thank you :), so in general with strong induction, you prove a base, then your induction hypothesis is that it works for all numbers between your base up until some value n, and you have to prove using this, that it also works for the n? thank you for all your help!
 Mentor P: 18,346 Yep, that is the idea behind strong induction. Good luck with your next problems!
 P: 29 thank you:)

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