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20 (twenty) is the natural number following 19 and preceding 21. A group of twenty units may also be referred to as a score.〔John H. Conway and Richard K. Guy, ''The Book of Numbers''. New York: Copernicus (1996): 11. ""Score" is related to "share" and comes from the Old Norse "skor" meaning a "notch" or "tally" on a stick used for counting. ... Often people counted in 20s; every 20th notch was larger, and so "score" also came to mean 20."〕 == In mathematics == *20 is a tetrahedral number as 1, 4, 10, 20. *20 is the basis for vigesimal number systems. *20 is the third composite number comprising the product of a squared prime and a prime, and also the second member of the (''2''2)''q'' family in this form. *20 has an aliquot sum of 22 (110% in abundance). Accordingly, 20 is the third abundant number and demonstrates an 8-member aliquot sequence; . *20 is the smallest primitive abundant number. *20 is the 4th composite number in the 7-aliquot tree. Two numbers have 20 as their aliquot sum; the discrete semiprime 34 and the squared prime 361. Only 2 other square primes are abundant 12 and 18. *An icosahedron has 20 faces. A dodecahedron has 20 vertices. *20 can be written as the sum of three Fibonacci numbers uniquely, i.e. 20 = 13 + 5 + 2. *The product of the number of divisors and the number of proper divisors of 20 is exactly 20. *20 is the number of quarter or half turns required to optimally solve a Rubik's Cube in the worst case. *20 is the only number with more than one digit that can be written from base 2 to base 20 using only the digits 0 to 9.〔Sequence A131646 in The On-Line Encyclopedia of Integer Sequences〕 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「20 (number)」の詳細全文を読む スポンサード リンク
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