Another formula I found out about:
$$f(n)=[\frac{\phi^n}{\sqrt5}]$$ (square brackets mean rounding down AKA truncation AKA conversion to `int`).

    from math import sqrt
    
    root_five = sqrt(5)
    phi = (root_five + 1) / 2
    
    def fibonacci(n):
        return int((phi ** n) / root_five)

Note: only supports the normal fibonacci sequence, may or may not work for negatives.