21723521 has 4 divisors (see below), whose sum is σ = 21734100. Its totient is φ = 21712944.

The previous prime is 21723469. The next prime is 21723539. The reversal of 21723521 is 12532712.

It is a happy number.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 13249600 + 8473921 = 3640^2 + 2911^2 .

It is a cyclic number.

It is not a de Polignac number, because 21723521 - 2¹⁴ = 21707137 is a prime.

It is a super-2 number, since 2×21723521² = 943822729274882, which contains 22 as substring.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21723571) 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, 1106 + ... + 6683.

It is an arithmetic number, because the mean of its divisors is an integer number (5433525).

Almost surely, 2^21723521 is an apocalyptic number.

It is an amenable number.

21723521 is a deficient number, since it is larger than the sum of its proper divisors (10579).

21723521 is an equidigital number, since it uses as much as digits as its factorization.

21723521 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10578.

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

The square root of 21723521 is about 4660.8498152161. The cubic root of 21723521 is about 279.0251858235.

The spelling of 21723521 in words is "twenty-one million, seven hundred twenty-three thousand, five hundred twenty-one".