21 (Number)
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21 is a natural number that falls between 20 and 22. It is a composite number, meaning it has divisors other than 1 and itself. Its divisors are 1, 3, 7, and 21.
In the decimal system, it is written as two digits: 2 and 1. The Somali word is 'labaatan iyo koow'. The Roman numeral is XXI.
Mathematical properties
21 is the product of 3 × 7, both of which are prime numbers. For this reason it is a semiprime number, with no other divisors. The sum of its proper divisors (1 + 3 + 7) equals 11, which is less than 21, making it a deficient number.
21 is also the sixth triangular number: 1 + 2 + 3 + 4 + 5 + 6 = 21. It also appears in the Fibonacci sequence (1, 1, 2, 3, 5, 8, 13, 21, 34...), where it is obtained from 8 + 13.
General usage
Many countries set 21 years as the age of legal majority for certain matters, such as alcohol consumption in the United States. The 21st century began on 1 January 2001.
In card games, the game Blackjack is built around achieving a card value of 21 without exceeding it. In chemistry, atomic number 21 denotes the element scandium.