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91 (ninety-one) is the natural number following 90 and preceding 92. ==In mathematics== 91 is the twenty-seventh distinct semiprime and the second of the form (7.q). The aliquot sum of 91 is 21 within the aliquot sequence (91,21,11,1,0) 91 being the fourth composite number in the 11-aliquot tree. 91 is a triangular number and a hexagonal number, one of the few such numbers to also be a centered hexagonal number, and it is also a centered nonagonal number and a centered cube number. It is a square pyramidal number, being the sum of the squares of the first six integers. 91 is the smallest positive integer expressible as a sum of two cubes in two different ways if negative roots are allowed (alternatively the sum of two cubes and the difference of two cubes): 91 = 63+(-5)3 = 43+33. (See 1729 for more details). This implies that 91 is the second cabtaxi number. The decimal equivalent of the fraction 1/91 can be obtained by using powers of 9. 91 is the smallest positive integer expressible as a sum of six distinct squares: 91 = 12+22+32+42+52+62. The only other ways to write 91 as a sum of distinct squares are: 91 = 12+42+52+72 and 91 = 12+32+92. 91 is the smallest pseudoprime satisfying the congruence 3''n'' = 3 mod ''n''.〔Friedman, Erich. (What's Special About This Number? )〕 91 is a repdigit in base 9 (111). 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「91 (number)」の詳細全文を読む スポンサード リンク
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