A couple of commenters have mentioned this, but with his definition of fib that's given right below in the 'overview' section, the function works: Fib(0) = 1

A couple of commenters have mentioned this, but with his definition of fib that's given right below in the 'overview' section, the function works: Fib(0) = 1 Fib(1) = 1 Fib(n) = Fib(n−1) + Fib(n−2) So n has a constraint of n > 1, for which the function works.