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81 may refer to: * 81 (number) * one of the years 81 BC, AD 81, 1981, 2081 * Nickname for the Hells Angels Motorcycle Club. "H" is the eighth letter of the alphabet, and "A" is the first. See also * * List of highways numbered {{Numberdis ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
81 (number)
81 (eighty-one) is the natural number following 80 and preceding 82. In mathematics 81 is: * the square of 9 and the fourth power of 3. * a perfect totient number like all powers of three. * a heptagonal number. * a centered octagonal number. * a tribonacci number. * an open meandric number. * the ninth member of the Mian-Chowla sequence. * a palindromic number in bases 8 (1218) and 26 (3326). * a Harshad number in bases 2, 3, 4, 7, 9, 10 and 13. * one of three non-trivial numbers (the other two are 1458 and 1729) which, when its digits (in decimal) are added together, produces a sum which, when multiplied by its reversed self, yields the original number: : 8 + 1 = 9 : 9 × 9 = 81 (although this case is somewhat degenerate, as the sum has only a single digit). The inverse of 81 is 0. recurring, missing only the digit "8" from the complete set of digits. This is an example of the general rule that, in base ''b'', :\frac = 0.\overline, omitting only the digit ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |