Forty is an octagonal number, and as the sum of the first four pentagonal numbers, 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 φx) = n.
Forty is the number of n-queens pro
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