21742721 has 48 divisors (see below), whose sum is σ = 31026240. Its totient is φ = 14958720.

The previous prime is 21742709. The next prime is 21742727. The reversal of 21742721 is 12724712.

It is not a de Polignac number, because 21742721 - 2¹⁸ = 21480577 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21742727) by changing a digit.

It is a polite number, since it can be written in 47 ways as a sum of consecutive naturals, for example, 203150 + ... + 203256.

It is an arithmetic number, because the mean of its divisors is an integer number (646380).

Almost surely, 2^21742721 is an apocalyptic number.

It is an amenable number.

21742721 is a deficient number, since it is larger than the sum of its proper divisors (9283519).

21742721 is a wasteful number, since it uses less digits than its factorization.

21742721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 174 (or 167 counting only the distinct ones).

The product of its digits is 1568, while the sum is 26.

The square root of 21742721 is about 4662.9090705267. The cubic root of 21742721 is about 279.1073656571.

Subtracting from 21742721 its reverse (12724712), we obtain a square (9018009 = 3003²).

The spelling of 21742721 in words is "twenty-one million, seven hundred forty-two thousand, seven hundred twenty-one".