Great use of recursion. Unfortunately, this solution should fail the updated test suite due to performance (time out) for cases n>36;

Thanks for the tip off.   By the way, it turns out that in the updated Test Suite there is an inherent imprecision (or 'tolerance') of about ±64 in the stated 'correct' result for n=70: see Solution 1528518.   On the other hand, you could enforce exact solutions even for n=70 as suggested in Solution 1528517 (like Problem 44628).   —DIV

In the code f(1), f(2) & f(3) are defined, but never used.

David Verrelli, yes, they are never used. I forgot to remove those lines. Thanks!