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51 (fifty-one) is the natural number 51 following 50 and preceding 52. ==In mathematics== Fifty-one is the 14th discrete semiprime and the 5th in the semiprime family having the prime factors (3.17). The aliquot sum of 51 is 21, itself a discrete semiprime. 51 is the 4th member of the 11-aliquot tree with the sequence (51, 21, 11, 1, 0). 51 is a pentagonal number as well as a centered pentagonal number (one of the few numbers to be both) and an 18-gonal number and a Perrin number. It is also the 6th Motzkin number, telling the number of ways to draw non-intersecting chords between any six points on a circle's boundary, no matter where the points may be located on the boundary. Since the greatest prime factor of 512 + 1 = 2602 is 1301, which is substantially more than 51 twice, 51 is a Størmer number. There are 51 different cyclic Gilbreath permutations on 10 elements, and therefore there are 51 different real periodic points of order 10 on the Mandelbrot set.〔.〕 Since 51 is the product of the distinct Fermat primes 3 and 17, a regular polygon with 51 sides is constructible with compass and straightedge, the angle is constructible, and the number is expressible in terms of square roots. 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「51 (number)」の詳細全文を読む スポンサード リンク
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