21293721 has 6 divisors (see below), whose sum is σ = 30757610. Its totient is φ = 14195808.

The previous prime is 21293717. The next prime is 21293747. The reversal of 21293721 is 12739212.

It can be written as a sum of positive squares in only one way, i.e., 21261321 + 32400 = 4611^2 + 180^2 .

It is not a de Polignac number, because 21293721 - 2² = 21293717 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21293711) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 1182976 + ... + 1182993.

Almost surely, 2^21293721 is an apocalyptic number.

It is an amenable number.

21293721 is a deficient number, since it is larger than the sum of its proper divisors (9463889).

21293721 is a wasteful number, since it uses less digits than its factorization.

21293721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2365975 (or 2365972 counting only the distinct ones).

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

The square root of 21293721 is about 4614.5120002011. The cubic root of 21293721 is about 277.1727430206.

The spelling of 21293721 in words is "twenty-one million, two hundred ninety-three thousand, seven hundred twenty-one".