Efficient solution to Project Euler 14 (Longest Collatz sequence)
n must be 0, 1, 2 ...


Should be:

n must be 1, 2, 3 ...


Or say that:

n must be a positive integer.


Your code doesn't give an infinite loop, but it doesn't make sense really that you return [0] for your function. If you "allowed" 0 the collatz funciton would return an infinite list of 0s. (Why?)