21756621 has 4 divisors (see below), whose sum is σ = 29008832. Its totient is φ = 14504412.

The previous prime is 21756613. The next prime is 21756649. The reversal of 21756621 is 12665712.

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 not a de Polignac number, because 21756621 - 2³ = 21756613 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 (21756671) 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, 3626101 + ... + 3626106.

It is an arithmetic number, because the mean of its divisors is an integer number (7252208).

Almost surely, 2^21756621 is an apocalyptic number.

It is an amenable number.

21756621 is a deficient number, since it is larger than the sum of its proper divisors (7252211).

21756621 is an equidigital number, since it uses as much as digits as its factorization.

21756621 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7252210.

The product of its digits is 5040, while the sum is 30.

The square root of 21756621 is about 4664.3993182402. The cubic root of 21756621 is about 279.1668302467.

Subtracting from 21756621 its reverse (12665712), we obtain a palindrome (9090909).

The spelling of 21756621 in words is "twenty-one million, seven hundred fifty-six thousand, six hundred twenty-one".