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15 (fifteen) is the natural number following 14 and preceding 16. In English, it is the smallest natural number with seven letters in its spelled name. In spoken English, the numbers 15 and 50 are often confused because they sound similar. When carefully enunciated, they differ in which syllable is stressed: 15 vs 50 . However, in dates such as 1500 ("fifteen hundred") or when contrasting numbers in the teens, the stress generally shifts to the first syllable: 15 . ==In mathematics== Fifteen is a triangular number, a hexagonal number, a pentatope number and the 5th Bell number (i.e. the number of partitions for a set of size 4). Fifteen is the double factorial of 5, and there are 15 perfect matchings of the complete graph ''K''6 and 15 rooted binary trees with four labeled leaves, both of these being among the types of object counted by double factorials. It is a composite number; its proper divisors being , and . With only two exceptions, all prime quadruplets enclose a multiple of 15, with 15 itself being enclosed by the quadruplet (11, 13, 17, 19). 15 is also the number of supersingular primes. 15 is the 4th discrete semiprime (3.5) and the first member of the (3.q) discrete semiprime family. It is thus the first odd discrete semiprime. The number proceeding 15; 14 is itself a discrete semiprime and this is the first such pair of discrete semiprimes. The next example is the pair commencing 21. The aliquot sum of 15 is 9, a square prime 15 has an aliquot sequence of 6 members (15,9,4,3,1,0). 15 is the fourth composite number in the 3-aliquot tree. The abundant 12 is also a member of this tree. Fifteen is the aliquot sum of the consecutive 4-power 16, and the discrete semiprime 33. 15 and 16 form a Ruth-Aaron pair under the second definition in which repeated prime factors are counted as often as they occur. Fifteen is the magic constant of the unique order-3 normal magic square: 15 is the smallest number that can be factorized using Shor's quantum algorithm. There are 15 solutions to Znám's problem of length 7. 15 is a repdigit in binary (1111) and quaternary (33). In hexadecimal, as well as all higher bases, 15 is represented as F. Because 15 is the product of distinct Fermat primes, 3 and 5, a regular polygon with 15 sides is constructible with compass and unmarked straightedge, and is expressible in terms of square roots (see here). If a positive definite quadratic form with integer matrix represents all positive integers up to 15, then it represents all positive integers via the 15 and 290 theorems. 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「15 (number)」の詳細全文を読む スポンサード リンク
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