21656621 has 4 divisors (see below), whose sum is σ = 24750432. Its totient is φ = 18562812.

The previous prime is 21656597. The next prime is 21656627. The reversal of 21656621 is 12665612.

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 not a de Polignac number, because 21656621 - 2¹⁴ = 21640237 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21656627) 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, 1546895 + ... + 1546908.

It is an arithmetic number, because the mean of its divisors is an integer number (6187608).

Almost surely, 2^21656621 is an apocalyptic number.

It is an amenable number.

21656621 is a deficient number, since it is larger than the sum of its proper divisors (3093811).

21656621 is an equidigital number, since it uses as much as digits as its factorization.

21656621 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3093810.

The product of its digits is 4320, while the sum is 29.

The square root of 21656621 is about 4653.6674784518. The cubic root of 21656621 is about 278.7384615868.

The spelling of 21656621 in words is "twenty-one million, six hundred fifty-six thousand, six hundred twenty-one".