# Fibonacci numbers with negative indices?

## Homework Statement

Let the Fibonacci sequence Fn be defined by its recurrence relation (1) Fn=F(n-1)+F(n-2) for n>=3. Show that there is a unique way to extend the definition of Fn to integers n<=0 such that (1) holds for all integers n, and obtain an explicit formula for the terms Fn with negative indices n.

## The Attempt at a Solution

So I know the solution uses induction, and I think the first few negative terms should be F-1=-1, F-2=-1, F-3=-2 etc. So for the negative integers, Fn=F(n+1) + F(n+2) for n<0, but if the formula is supposed to extend to all integers n, that formula doesn't work...am I thinking about this problem wrong?

## Answers and Replies

Ok, so i worked on this a bit more, and found that the formula I'm trying to prove is Fn=F(n+2)-F(n+1), since this generates the negative terms...would the base case be n=1 and the induction hypothesis prove n=k-1?