(12C) Check if given number is Fibonacci number

[Albert Chan]

Nice formula:

If (5 x^2 +/- 4) is perfect square, x is Fibonacci number.

However, there is a weakness if done on the HP-12C.
For x > 44721, the expression overflow 10-digits. Rounding errors kill the algorithm.

So, above code has a safe domain upto 44721 (only 23 Fibonacci numbers in that range)
It might be better to generate the Fibonacci numbers directly, and test the input.