21643621 has 4 divisors (see below), whose sum is σ = 22584672. Its totient is φ = 20702572.

The previous prime is 21643607. The next prime is 21643691. The reversal of 21643621 is 12634612.

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 21643621 - 2¹⁹ = 21119333 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21643691) 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, 470491 + ... + 470536.

It is an arithmetic number, because the mean of its divisors is an integer number (5646168).

Almost surely, 2^21643621 is an apocalyptic number.

It is an amenable number.

21643621 is a deficient number, since it is larger than the sum of its proper divisors (941051).

21643621 is an equidigital number, since it uses as much as digits as its factorization.

21643621 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 941050.

The product of its digits is 1728, while the sum is 25.

The square root of 21643621 is about 4652.2705209392. The cubic root of 21643621 is about 278.6826768717.

Subtracting from 21643621 its reverse (12634612), we obtain a palindrome (9009009).

The spelling of 21643621 in words is "twenty-one million, six hundred forty-three thousand, six hundred twenty-one".