The Mertens function of n is the sum of Mobius k for k from 1 to n. There should be a more efficient computation of it than +/ @: (mobius"0) @: >: @ i. http://en.wikipedia.org/wiki/Mertens_function http://en.wikipedia.org/wiki/Mobius_function