Hi mathgeeks,

Given an extremely large fibonacci number X, and it's position in the fibonacci sequence N, is there any way to determine the N-1th fibo num, WITHOUT doing the bruteforce dance of counting from 1 upwards?

I've tried attacking the problem by dividing the number by two, and determining the difference between the N-1th, and N-2th, but that would require calculating Fib(n-3)/2, which makes the problem circular.

Also would be interested, if this can be proven not to be possible, thanks