[travisjungroth]

Properties of the Fibonacci function:

It is non-decreasing monotonic. fib(n) <= fib(n+1)

It is increasing monotonic after 1. fib(n) < fib(n+1)

Its domain and codomain are non-negative integers.

fib(n) + fib(n+1) == fib(n+2) Notice this is like the recursive solution except going the other way (addition not subtraction) and is missing the base case.

[xtagon]

It also converges closer and closer to the golden ratio. Implementing a property test for that would be interesting.

[travisjungroth]

    a, b, c = fib(n), fib(n+1), fib(n+2)
    assert abs(c / b - phi) < abs(b / a - phi)