21904 has 15 divisors (see below), whose sum is σ = 43617. Its totient is φ = 10656.

The previous prime is 21893. The next prime is 21911. The reversal of 21904 is 40912.

21904 = T₁₄₇ + T₁₄₈.

The square root of 21904 is 148.

It is a perfect power (a square), and thus also a powerful number.

It can be written as a sum of positive squares in only one way, i.e., 19600 + 2304 = 140^2 + 48^2 .

It is a Harshad number since it is a multiple of its sum of digits (16).

It is an Ulam number.

It is a Duffinian number.

It is an unprimeable number.

It is a polite number, since it can be written in 2 ways as a sum of consecutive naturals, for example, 574 + ... + 610.

2²¹⁹⁰⁴ is an apocalyptic number.

21904 is the 148-th square number.

It is an amenable number.

21904 is a deficient number, since it is larger than the sum of its proper divisors (21713).

21904 is an equidigital number, since it uses as much as digits as its factorization.

21904 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 82 (or 39 counting only the distinct ones).

The product of its (nonzero) digits is 72, while the sum is 16.

The cubic root of 21904 is about 27.9795769439.

Multiplying 21904 by its sum of digits (16), we get a square (350464 = 592²).

21904 divided by its sum of digits (16) gives a square (1369 = 37²).

The spelling of 21904 in words is "twenty-one thousand, nine hundred four".