# 21 (number)

21 (twenty-one) is the natural number following 20 and preceding 22.

 ← 20 21 22 →
Cardinaltwenty-one
Ordinal21st
(twenty-first)
Factorization3 × 7
Divisors1, 3, 7, 21
Greek numeralΚΑ´
Roman numeralXXI
Binary101012
Ternary2103
Octal258
Duodecimal1912

## In mathematics

21 is:

• a composite number, its proper divisors being 1, 3 and 7, and a deficient number as the sum of these divisors is less than the number itself.
• a Fibonacci number as it is the sum of the preceding terms in the sequence, 8 and 13.
• the fifth Motzkin number.
• a triangular number, because it is the sum of the first six natural numbers (1 + 2 + 3 + 4 + 5 + 6 = 21).
• an octagonal number.
• a Padovan number, preceded by the terms 9, 12, 16 (it is the sum of the first two of these) in the padovan sequence.
• a Blum integer, since it is a semiprime with both its prime factors being Gaussian primes.
• the sum of the divisors of the first 5 positive integers (i.e., 1 + (1 + 2) + (1 + 3) + (1 + 2 + 4) + (1 + 5))
• the smallest non-trivial example of a Fibonacci number whose digits are Fibonacci numbers and whose digit sum is also a Fibonacci number.
• a Harshad number.* a repdigit in base 4 (1114).
• the smallest natural number that is not close to a power of 2, 2n, where the range of closeness is ±n.
• the smallest number of differently sized squares needed to square the square.
• the largest n with this property: for any positive integers a,b such that a + b = n, at least one of ${\tfrac {a}{b}}$  and ${\tfrac {b}{a}}$  is a terminating decimal. See a brief proof below.

Note that a necessary condition for n is that for any a coprime to n, a and n - a must satisfy the condition above, therefore at least one of a and n - a must only have factor 2 and 5.

Let $A(n)$  denote the quantity of the numbers smaller than n that only have factor 2 and 5 and that are coprime to n, we instantly have ${\frac {\varphi (n)}{2}} .

We can easily see that for sufficiently large n, $A(n)\sim {\frac {\log _{2}(n)\log _{5}(n)}{2}}={\frac {\ln ^{2}(n)}{2\ln(2)\ln(5)}}$ , but $\varphi (n)\sim {\frac {n}{e^{\gamma }\;\ln \ln n}}$ , $A(n)=o(\varphi (n))$  as n goes to infinity, thus ${\frac {\varphi (n)}{2}}  fails to hold for sufficiently large n.

In fact, For every n > 2, we have

$A<1+\log _{2}(n)+{\frac {3\log _{5}(n)}{2}}+{\frac {\log _{2}(n)\log _{5}(n)}{2}}$

and

$\varphi (n)>{\frac {n}{e^{\gamma }\;\log \log n+{\frac {3}{\log \log n}}}}$

so ${\frac {\varphi (n)}{2}}<$  fails to hold when n > 273 (actually, when n > 33).

Just check a few numbers to see that '= 2, 3, 4, 5, 6, 7, 8, 9, 11, 12, 15, 21.

## Age 21

• In thirteen countries, 21 is the age of majority. See also: Coming of age.
• In eight countries, 21 is the minimum age to purchase tobacco products.
• In seventeen countries, 21 is the drinking age.
• In nine countries, it is the voting age.
• In the United States:
• 21 is the minimum age at which a person may gamble or enter casinos in most states (since alcohol is usually provided).
• 21 is the minimum age to purchase a handgun or handgun ammunition under federal law.
• 21 is the age at which one can purchase multiple tickets to an R-rated film.
• In some states, 21 is the minimum age to accompany a learner driver, provided that the person supervising the learner has held a full driver license for a specified amount of time. See also: List of minimum driving ages.

## In sports

• Twenty-one is a variation of street basketball, in which each player, of which there can be any number, plays for himself only (i.e. not part of a team); the name comes from the requisite number of baskets.
• In three-on-three basketball games held under FIBA rules, branded as 3x3, the game ends by rule once either team has reached 21 points.
• In badminton, and table tennis (before 2001), 21 points are required to win a game.
• In AFL Women's, the top-level league of women's Australian rules football, each team is allowed a squad of 21 players (16 on the field and five interchanges).

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