21023721 has 6 divisors (see below), whose sum is σ = 30367610. Its totient is φ = 14015808.

The previous prime is 21023701. The next prime is 21023731. The reversal of 21023721 is 12732012.

It can be written as a sum of positive squares in only one way, i.e., 17943696 + 3080025 = 4236^2 + 1755^2 .

It is not a de Polignac number, because 21023721 - 2⁹ = 21023209 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21023694 and 21023703.

It is not an unprimeable number, because it can be changed into a prime (21023701) 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, 1167976 + ... + 1167993.

Almost surely, 2^21023721 is an apocalyptic number.

It is an amenable number.

21023721 is a deficient number, since it is larger than the sum of its proper divisors (9343889).

21023721 is a wasteful number, since it uses less digits than its factorization.

21023721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2335975 (or 2335972 counting only the distinct ones).

The product of its (nonzero) digits is 168, while the sum is 18.

The square root of 21023721 is about 4585.1631377738. The cubic root of 21023721 is about 275.9962586134.

Adding to 21023721 its reverse (12732012), we get a palindrome (33755733).

The spelling of 21023721 in words is "twenty-one million, twenty-three thousand, seven hundred twenty-one".