21318921 has 24 divisors (see below), whose sum is σ = 33338760. Its totient is φ = 13049856.

The previous prime is 21318911. The next prime is 21318923. The reversal of 21318921 is 12981312.

It can be written as a sum of positive squares in 4 ways, for example, as 2134521 + 19184400 = 1461^2 + 4380^2 .

It is not a de Polignac number, because 21318921 - 2⁵ = 21318889 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21318923) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 29715 + ... + 30423.

It is an arithmetic number, because the mean of its divisors is an integer number (1389115).

Almost surely, 2^21318921 is an apocalyptic number.

It is an amenable number.

21318921 is a deficient number, since it is larger than the sum of its proper divisors (12019839).

21318921 is a wasteful number, since it uses less digits than its factorization.

21318921 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 985 (or 982 counting only the distinct ones).

The product of its digits is 864, while the sum is 27.

The square root of 21318921 is about 4617.2417090726. The cubic root of 21318921 is about 277.2820397091.

The spelling of 21318921 in words is "twenty-one million, three hundred eighteen thousand, nine hundred twenty-one".