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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).
Computable number: A real number whose digits can be computed by some algorithm. Period: A number which can be computed as the integral of some algebraic function over an algebraic domain. Definable number: A real number that can be defined uniquely using a first-order formula with one free variable in the language of set theory.
21, also "Counting to 21 game" or "21 game", a Nim-style counting game Twenty-one (basketball) , a basketball variation Twenty-One (game show) , an American game show from 1956 to 1958 and in 2000
A list of articles about numbers (not about numerals). Topics include powers of ten, notable integers, prime and cardinal numbers, and the myriad system.
All prime numbers from 31 to 6,469,693,189 for free download. Lists of Primes at the Prime Pages. The Nth Prime Page Nth prime through n=10^12, pi(x) through x=3*10^13, Random primes in same range.
English number words include numerals and various words derived from them, ... If a number is in the range 21 to 99, and the second digit is not zero, ...
C 21 H 18 O 12: Several molecules: 27740-01-8 C 21 H 20 Cl 2 O 3: permethrin: 52645-53-1 C 21 H 20 O 6: curcumin: 458-37-7 C 21 H 20 O 9: Several molecules - C 21 H 20 O 10: Several molecules - C 21 H 20 O 11: Several molecules - C 21 H 20 O 12: Several molecules - C 21 H 21 N: cyproheptadine: 129-03-3 C 21 H 21 NO 6: hydrastine: 118-08-1 C 21 ...
Prohibiting calling out a series of numbers that sum to 21 (e.g. "6, 7, 8" or "10, 11") Examples of new rules that could be instated by a player forced to call 21: Swapping two numbers around (e.g. 3 and 13 are switched) Replacing a number with a humorous phrase (e.g. replacing 5 with "skin" - causing the sequence 1, 2, 3, "foreskin")