Seriously? The discussion is about language features, not optimal algorithms. One could imagine the question was actually "sum BusyBeaver(n) for n divisible by

Seriously? The discussion is about language features, not optimal algorithms. One could imagine the question was actually "sum BusyBeaver(n) for n divisible by 3 and 5 between 1 and 100", i.e. no closed form. In Python: sum(BusyBeaver(x) for x in xrange(1,101) if x % 3 == 0 and x % 5 == 0) (And yes, if one was being really pedantic, one could replace the condition with x % 15 == 0.)