In mathematics Forty is a composite number an octagonal number and as the sum of the first four pentagonal numbers 1 5 12 22 40 it is a pentagonal pyramidal number Adding up some subsets of its divisors e g 1 4 5 10 and 20 gives 40 hence 40 is a semiperfect number Given 40 the Mertens function returns 0 40 is the smallest number n with exactly 9 solutions to the equation φ x …