21666621 has 16 divisors (see below), whose sum is σ = 30453120. Its totient is φ = 13675200.

The previous prime is 21666611. The next prime is 21666637. The reversal of 21666621 is 12666612.

It is not a de Polignac number, because 21666621 - 2⁷ = 21666493 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21666611) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 5415 + ... + 8523.

It is an arithmetic number, because the mean of its divisors is an integer number (1903320).

Almost surely, 2^21666621 is an apocalyptic number.

It is an amenable number.

21666621 is a deficient number, since it is larger than the sum of its proper divisors (8786499).

21666621 is a wasteful number, since it uses less digits than its factorization.

21666621 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3236.

The product of its digits is 5184, while the sum is 30.

The square root of 21666621 is about 4654.7417758668. The cubic root of 21666621 is about 278.7813577169.

Subtracting from 21666621 its reverse (12666612), we obtain a palindrome (9000009).

The spelling of 21666621 in words is "twenty-one million, six hundred sixty-six thousand, six hundred twenty-one".