Collatz sequence in Python

#!/usr/bin/env python3
# http://stackoverflow.com/questions/43922426/how-to-write-code-to-find-the-n-th-number-in-the-collatz-sequance

# The following is a direct implementation based on what is shown here:
#   https://en.wikipedia.org/wiki/Collatz_conjecture#Statement_of_the_problem

def f(a):
    if a % 2 == 0:
        return a//2
    else:
        return 3*a + 1

def collatz(a, n):
    for _ in range(n):
        a = f(a)
    return a

print(collatz(11, 5))  # -> 13