# Proof a recursive formula with induction

• Sep 27th 2011, 03:27 PM
infnty
Proof a recursive formula with induction
Hi all!

I'm new here and I could use some help on a homework question.

Problem
Given the following recursive definition:

$\\ a_0=1 \\ a_1=2 \\ a_n=\frac{(a_{n-1})^2}{a_{n-2}}$

I have to proof, using induction, that

$\\ a_n=2^n$

My current proof is as follows:

Proof
Base step
The formula is correct for the cases n=0 and n=1 (Can be easily verified).

Induction step
Assume the formula is correct for n, we can fill this in:

$a_n = \frac{(2^{n-1})^2}{2^{n-2}}$

$a_n = \frac{2^{2n-2}}{2^{n-2}}$

$a_n = \frac{2^{n-2} \times 2^n}{2^{n-2}}$

$a_n = 2^n$

However, I think I am missing some important induction steps. I don't see the connection between the base step and the induction step. (Worried)

Any help is appreciated, thanks in advance! (Happy)
• Sep 27th 2011, 03:50 PM
emakarov
Re: Proof a recursive formula with induction
In the induction step, you should fix an arbitrary n >= 2 and assume that the claim holds for n - 1 and n - 2. Then you need to prove it for n. In other words, you prove the following statement where P(n) denotes the claim for n: "For all n >= 2, if P(n - 2) and P(n - 1), then P(n)." Since you proved P(0) and P(1), the induction step gives P(2), then from P(1) and P(2) you get P(3) and so on.

The calculations you did are correct.