Determines if a number is a Fibonacci number by a) determining element in the Fibonacci sequence it would be, based on its size, and b) comparing it to that actual element in the Fibonacci sequence. Couldn't have done this without a recursive formula courtesy of the late Edsgar Dijkstra, http://www.cs.utexas.edu/users/EWD/ewd06xx/EWD654.PDF foun…

fibonaccicheck.rb

def is_fibonacci?(num)
    phi = (1 + 5.0**0.5)/2
    which_fib = ((Math.log(num)+Math.log(5.0**0.5)) / Math.log(phi)).round
        
    num == fibonacci(which_fib)
end

def fibonacci(n)
  if n==0
		0
	elsif n==1
		1
	elsif n.odd? 
		(fibonacci(n/2))**2 + (fibonacci(n/2 + 1))**2 
	else 
		fibonacci(n/2) * (fibonacci(n/2) + fibonacci(n/2 - 1) * 2)
	end
end

Revision 2 correctly takes the variable "num" as the argument instead of the original "i"