21523721 has 4 divisors (see below), whose sum is σ = 21826944. Its totient is φ = 21220500.

The previous prime is 21523681. The next prime is 21523727. The reversal of 21523721 is 12732512.

It is a happy number.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21523721 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21523727) 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, 151505 + ... + 151646.

It is an arithmetic number, because the mean of its divisors is an integer number (5456736).

Almost surely, 2^21523721 is an apocalyptic number.

It is an amenable number.

21523721 is a deficient number, since it is larger than the sum of its proper divisors (303223).

21523721 is an equidigital number, since it uses as much as digits as its factorization.

21523721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 303222.

The product of its digits is 840, while the sum is 23.

The square root of 21523721 is about 4639.3664438154. The cubic root of 21523721 is about 278.1671139527.

It can be divided in two parts, 215 and 23721, that multiplied together give a palindrome (5100015).

The spelling of 21523721 in words is "twenty-one million, five hundred twenty-three thousand, seven hundred twenty-one".