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15 is: The eighth composite number and the sixth semiprime and the first odd and fourth discrete semiprime; [1] its proper divisors are 1, 3, and 5, so the first of the form (3.q), [2] where q is a higher prime. a deficient number, a lucky number, a bell number (i.e., the number of partitions for a set of size 4), [3] a pentatope number, [4 ...
13 (thirteen) is the natural number following 12 and preceding 14.. Strikingly folkloric aspects of the number 13 have been noted in various cultures around the world: one theory is that this is due to the cultures employing lunar-solar calendars (there are approximately 12.41 lunations per solar year, and hence 12 "true months" plus a smaller, and often portentous, thirteenth month).
Twelve is a superior highly composite number, divisible by the numbers from 1 to 4, and 6 . It is the number of years required for an orbital period of Jupiter. It is central to many systems of timekeeping, including the Western calendar and units of time of day, and frequently appears in the world's major religions.
As a biprime with proper divisors 1, 3 and 7, twenty-one has a prime aliquot sum of 11 within an aliquot sequence containing only one composite number (21, 11, 1, 0); it is the second composite number with an aliquot sum of 11, following 18. 21 is the first member of the second cluster of consecutive discrete semiprimes (21, 22), where the next such cluster is (33, 34, 35).
Integer properties. Twenty is a pronic number, as it is the product of consecutive integers, namely 4 and 5. [ 3] It is also the second pronic sum number (or pronic pyramid) after 2, being the sum of the first three pronic numbers: 2 + 6 + 12. It is the third composite number to be the product of a squared prime and a prime (and also the second ...
In mathematics. 30 is a square pyramidal number. 30 is an even, composite, pronic number. With 2, 3, and 5 as its prime factors, it is a regular number and the first sphenic number, the smallest of the form , where r is a prime greater than 3. It has an aliquot sum of 42, which is the second sphenic number. It is also:
14 is the third distinct semiprime, [1] being the third of the form (where is a higher prime). More specifically, it is the first member of the second cluster of two discrete semiprimes (14, 15); the next such cluster is (21, 22), members whose sum is the fourteenth prime number, 43.
65 is the nineteenth distinct semiprime, [1] (5.13); and the third of the form (5.q), where q is a higher prime. 65 has a prime aliquot sum of 19 within an aliquot sequence of one composite numbers (65, 19, 1 ,0) to the prime; as the first member' of the 19 -aliquot tree. It is an octagonal number. [2] It is also a Cullen number. [3]