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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).
Scandium is a chemical element with the symbol Sc and atomic number 21. It is a silvery-white metallic d-block element. Historically, it has been classified as a rare-earth element, [9] together with yttrium and the lanthanides. It was discovered in 1879 by spectral analysis of the minerals euxenite and gadolinite from Scandinavia. [10]
For all but the first seven weeks of his major league career, Clemente wore number 21, so chosen because his full name of Roberto Clemente Walker had that many letters. [20] For his first few weeks, Clemente wore the number 13, as his teammate Earl Smith was wearing number 21. It was later reassigned to Clemente. [21]
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.
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]
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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 ...
In some countries, the number 20 is used as an index in measuring visual acuity. 20/20 indicates normal vision at 20 feet, although it is commonly used to mean "perfect vision" in countries using the Imperial system.