title: Test if a number is fibonacci
body: <p>I know how to make the list of the Fibonacci numbers, but i don't know how can i test if a given number belongs to the fibonacci list - one way that comes in mind is generate the list of fib. numbers up to that number and see if it belongs to the array, but there's got to be another, simpler and faster method.</p>
  
  <p>Any ideas ?</p>