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85 (eighty-five) is the natural number following 84 and preceding 86. ==In mathematics== 85 is the product of two prime numbers (5 and 17), and is therefore a biprime; specifically, the 24th biprime not counting perfect squares. Together with 86 and 87, it forms the second cluster of three consecutive biprimes. The aliquot sum of 85 is 23 within the aliquot sequence (23,1,0), 85 being the second composite number in the 23-aliquot tree. 85 is an octahedral number, a centered triangular number, a centered square number, and a decagonal number. 85 can be expressed as a sum of two squares in two ways, 85= 92 + 22 = 72 + 62, the lowest number to have this property with all squares greater than 1.〔. Note that the two smaller numbers on this list, 50 and 65, have 1 as one of the squares in their representations as sums of two squares.〕 It is also the length of the hypotenuse of four pythagorean triangles. In base 10 it is a Smith number. 85 is 1111 (a repunit) in base 4. 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「85 (number)」の詳細全文を読む スポンサード リンク
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