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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).
Twenty-one, XXI or 21 may refer to: 21 (number) One of the years 21 BC, AD 21, 1921, 2021; Films.
22 is a semiprime, [1] a Smith number, and an Erdős–Woods number. [2] [3] = … is a commonly used approximation of the irrational number π, the ratio of the circumference of a circle to its diameter.
#21. TIL there were just 5 surviving longbows from medieval England known to exist before 137 whole longbows (and 3,500 arrows) were recovered from the wreck of the Mary Rose in 1980 (a ship of ...
Twenty-eight is a harmonic divisor number, [4] a happy number, [5] the 7th triangular number, [6] a hexagonal number, [7] a Leyland number of the second kind [8] (), and a centered nonagonal number. [9] It appears in the Padovan sequence, preceded by the terms 12, 16, 21 (it is the sum of the first two of these). [10]
73 has 37 as the mirroring of its decimal digits. 73 is the 21st prime number, and 37 the 12th. The "mirror property" is fulfilled when 73 has a mirrored permutation of its digits (37) that remains prime. Similarly, their respective prime indices (21 and 12) in the list of prime numbers are also permutations of the same digits (1, and 2).
Though the number of women representatives continues to rise, it's important to keep in mind that the United States population is 50.4% women, according to Census data. 26. Women outnumber men as ...
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