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40 (forty) is the natural number following 39 and preceding 41. 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. == In mathematics == 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「40 (number)」の詳細全文を読む スポンサード リンク
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