21508631 has 2 divisors, whose sum is σ = 21508632. Its totient is φ = 21508630.

The previous prime is 21508609. The next prime is 21508649. The reversal of 21508631 is 13680512.

It is a strong prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21508631 is a prime.

It is a super-2 number, since 2×21508631² = 925242414988322, which contains 22 as substring.

It is a Sophie Germain prime.

It is a Chen prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21508631.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21508601) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10754315 + 10754316.

It is an arithmetic number, because the mean of its divisors is an integer number (10754316).

Almost surely, 2^21508631 is an apocalyptic number.

21508631 is a deficient number, since it is larger than the sum of its proper divisors (1).

21508631 is an equidigital number, since it uses as much as digits as its factorization.

21508631 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 1440, while the sum is 26.

The square root of 21508631 is about 4637.7398590262. The cubic root of 21508631 is about 278.1020923101.

It can be divided in two parts, 215086 and 31, that multiplied together give a palindrome (6667666).

The spelling of 21508631 in words is "twenty-one million, five hundred eight thousand, six hundred thirty-one".