21 has 4 divisors (see below), whose sum is σ = 32. Its totient is φ = 12.

The previous prime is 19. The next prime is 23. The reversal of 21 is 12.

It is the 7-th Fibonacci number F_{7}.

21 is nontrivially palindromic in base 2, base 4 and base 6.

21 is digitally balanced in base 3, because in such base it contains all the possibile digits an equal number of times.

21 is an esthetic number in base 2, base 3, base 9 and base 10, because in such bases it adjacent digits differ by 1.

21 is a nontrivial binomial coefficient, being equal to C(7, 2).

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It is an interprime number because it is at equal distance from previous prime (19) and next prime (23).

It is the 5-th Motzkin number.

It is not a de Polignac number, because 21 - 2^{1} = 19 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (3), and also a Moran number because the ratio is a prime number: 7 = 21 / (2 + 1).

21 is an idoneal number.

It is the 6-th Jacobsthal number.

It is a D-number.

It is an alternating number because its digits alternate between even and odd.

It is the 5-th Hogben number.

It is a Duffinian number.

21 is an undulating number in base 2.

21 is strictly pandigital in base 3.

It is a Curzon number.

21 is a lucky number.

21 is a nontrivial repdigit in base 4 and base 6.

It is a plaindrome in base 4, base 6, base 8, base 9, base 11, base 12, base 13, base 14, base 15 and base 16.

It is a nialpdrome in base 3, base 4, base 5, base 6, base 7 and base 10.

It is a zygodrome in base 4 and base 6.

It is a congruent number.

It is a panconsummate number.

It is a nontrivial repunit in base 4.

It is a pernicious number, because its binary representation contains a prime number (3) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1 + ... + 6.

It is an arithmetic number, because the mean of its divisors is an integer number (8).

It is a 2-hyperperfect number.

21 is the 6-th triangular number and also the 3-rd octagonal number.

It is an amenable number.

21 is a deficient number, since it is larger than the sum of its proper divisors (11).

21 is an equidigital number, since it uses as much as digits as its factorization.

With its predecessor (20) it forms an eRAP, since the sums of their prime factors are consecutive (9 and 10).

21 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 10.

The product of its digits is 2, while the sum is 3.

The square root of 21 is about 4.5825756950. The cubic root of 21 is about 2.7589241764.

The spelling of 21 in words is "twenty-one", and is thus an aban number, an iban number, and an uban number.

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