21

21 (twenty-one) is the natural number following 20 and preceding 22. The current century is the 21st century AD, under the Gregorian calendar. In mathematics Twenty-one is the fifth distinct semiprime, and the second of the form where is a higher prime. 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 member of the 11-aliquot tree, following 18. 21 is the first member of the second cluster of two discrete semiprimes (21, 22), where the next such cluster is (38, 39). 21 is the smallest natural number that is not close to a power of two , where the range of nearness is . 21 is a Harshad number in base ten, and a repdigit in quaternary (1114).