[intv] if n is a fibonacci number.

N is a Fibonacci number if and only if (5*N*N + 4) or (5*N*N - 4) is a 
perfect square!

3 is a Fibonacci number since (5*3*3 + 4) is 49 which is 7*7
5 is a Fibonacci number since (5*5*5 - 4) is 121 which is 11*11
4 is not a Fibonacci number since neither (5*4*4 + 4) = 84 nor (5*4*4 - 4) 
= 76 are perfect squares.