21320991 has 24 divisors (see below), whose sum is σ = 32386640. Its totient is φ = 13499136.

The previous prime is 21320987. The next prime is 21321019. The reversal of 21320991 is 19902312.

It is not a de Polignac number, because 21320991 - 2² = 21320987 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21320911) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 13575 + ... + 15063.

Almost surely, 2^21320991 is an apocalyptic number.

21320991 is a deficient number, since it is larger than the sum of its proper divisors (11065649).

21320991 is a wasteful number, since it uses less digits than its factorization.

21320991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1575 (or 1572 counting only the distinct ones).

The product of its (nonzero) digits is 972, while the sum is 27.

The square root of 21320991 is about 4617.4658634364. The cubic root of 21320991 is about 277.2910138225.

The spelling of 21320991 in words is "twenty-one million, three hundred twenty thousand, nine hundred ninety-one".