21721928 has 16 divisors (see below), whose sum is σ = 42133500. Its totient is φ = 10486336.

The previous prime is 21721927. The next prime is 21721933. The reversal of 21721928 is 82912712.

It is a happy number.

It can be written as a sum of positive squares in 2 ways, for example, as 21696964 + 24964 = 4658^2 + 158^2 .

It is a junction number, because it is equal to n+sod(n) for n = 21721894 and 21721903.

It is not an unprimeable number, because it can be changed into a prime (21721927) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 46583 + ... + 47046.

Almost surely, 2^21721928 is an apocalyptic number.

It is an amenable number.

21721928 is a deficient number, since it is larger than the sum of its proper divisors (20411572).

21721928 is a wasteful number, since it uses less digits than its factorization.

21721928 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 93664 (or 93660 counting only the distinct ones).

The product of its digits is 4032, while the sum is 32.

The square root of 21721928 is about 4660.6789205007. The cubic root of 21721928 is about 279.0183652903.

The spelling of 21721928 in words is "twenty-one million, seven hundred twenty-one thousand, nine hundred twenty-eight".