21392721 has 16 divisors (see below), whose sum is σ = 36220800. Its totient is φ = 12224304.

The previous prime is 21392719. The next prime is 21392729. The reversal of 21392721 is 12729312.

It is not a de Polignac number, because 21392721 - 2¹ = 21392719 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 (21392729) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 56406 + ... + 56783.

It is an arithmetic number, because the mean of its divisors is an integer number (2263800).

Almost surely, 2^21392721 is an apocalyptic number.

21392721 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

21392721 is a deficient number, since it is larger than the sum of its proper divisors (14828079).

21392721 is a wasteful number, since it uses less digits than its factorization.

21392721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 113205 (or 113199 counting only the distinct ones).

The product of its digits is 1512, while the sum is 27.

The square root of 21392721 is about 4625.2265890440. The cubic root of 21392721 is about 277.6016282256.

The spelling of 21392721 in words is "twenty-one million, three hundred ninety-two thousand, seven hundred twenty-one".