21057921 has 32 divisors (see below), whose sum is σ = 32503680. Its totient is φ = 13462848.

The previous prime is 21057919. The next prime is 21057923. The reversal of 21057921 is 12975012.

It is an interprime number because it is at equal distance from previous prime (21057919) and next prime (21057923).

It is not a de Polignac number, because 21057921 - 2¹ = 21057919 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is not an unprimeable number, because it can be changed into a prime (21057923) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 106795 + ... + 106991.

It is an arithmetic number, because the mean of its divisors is an integer number (1015740).

Almost surely, 2^21057921 is an apocalyptic number.

It is an amenable number.

21057921 is a deficient number, since it is larger than the sum of its proper divisors (11445759).

21057921 is a wasteful number, since it uses less digits than its factorization.

21057921 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 350 (or 344 counting only the distinct ones).

The product of its (nonzero) digits is 1260, while the sum is 27.

The square root of 21057921 is about 4588.8910425069. The cubic root of 21057921 is about 276.1458350273.

The spelling of 21057921 in words is "twenty-one million, fifty-seven thousand, nine hundred twenty-one".