21391904 has 24 divisors (see below), whose sum is σ = 42508620. Its totient is φ = 10596096.

The previous prime is 21391889. The next prime is 21391919. The reversal of 21391904 is 40919312.

It is a happy number.

It is an interprime number because it is at equal distance from previous prime (21391889) and next prime (21391919).

It can be written as a sum of positive squares in 2 ways, for example, as 18079504 + 3312400 = 4252^2 + 1820^2 .

It is a super-2 number, since 2×21391904² = 915227113490432, which contains 22 as substring.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 422 + ... + 6554.

Almost surely, 2^21391904 is an apocalyptic number.

It is an amenable number.

21391904 is a deficient number, since it is larger than the sum of its proper divisors (21116716).

21391904 is a wasteful number, since it uses less digits than its factorization.

21391904 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 6252 (or 6244 counting only the distinct ones).

The product of its (nonzero) digits is 1944, while the sum is 29.

The square root of 21391904 is about 4625.1382682034. The cubic root of 21391904 is about 277.5980942600.

The spelling of 21391904 in words is "twenty-one million, three hundred ninety-one thousand, nine hundred four".