(12C) Check if given number is Fibonacci number

[Albert Chan]

Hi, stored

To extend your algorithm for full ten digits (your code take care of eight), I use Binet formula.
Instead of guessing n from X, do it in reverse, with HP12C correction to match actual Fib(n):

X = phi^n / sqrt(5)
X = X - X * 7.7e-9
X = INT(X + 0.5)

For n>40, if your code think the number may be Fibonacci, above can check it.
note: eps = 2e-8 may cover this range, but need actual confirmation.