Despite being related to the word "four" (4), the modern spelling of 40 is "forty". The archaic form "fourty" is now considered a misspelling. The modern spelling possibly reflects a pronunciation change due to the horse–hoarse merger.

Forty is a composite number, 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 φ(x) = n.

Forty is the number of n-queens problem solutions for n = 7.

Since 40² + 1 = 1601 is prime, 40 is a Størmer number.

40 is a repdigit in base 3 (1111, i.e. 3⁰ + 3¹ + 3² + 3³) and a Harshad number in base 10.

The number 40 is used in Jewish, Christian, Islamic, and other Middle Eastern traditions to represent a large, approximate number, similar to "umpteen".

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