21312531 has 16 divisors (see below), whose sum is σ = 32593920. Its totient is φ = 13749480.

The previous prime is 21312523. The next prime is 21312539. The reversal of 21312531 is 13521312.

21312531 is a `hidden beast` number, since 2 + 131 + 2 + 531 = 666.

It is an interprime number because it is at equal distance from previous prime (21312523) and next prime (21312539).

It is not a de Polignac number, because 21312531 - 2³ = 21312523 is a prime.

It is a super-2 number, since 2×21312531² = 908447955251922, which contains 22 as substring.

It is not an unprimeable number, because it can be changed into a prime (21312539) 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, 11895 + ... + 13568.

It is an arithmetic number, because the mean of its divisors is an integer number (2037120).

Almost surely, 2^21312531 is an apocalyptic number.

21312531 is a deficient number, since it is larger than the sum of its proper divisors (11281389).

21312531 is a wasteful number, since it uses less digits than its factorization.

21312531 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 25503 (or 25497 counting only the distinct ones).

The product of its digits is 180, while the sum is 18.

The square root of 21312531 is about 4616.5496856419. The cubic root of 21312531 is about 277.2543333461.

Adding to 21312531 its reverse (13521312), we get a palindrome (34833843).

The spelling of 21312531 in words is "twenty-one million, three hundred twelve thousand, five hundred thirty-one".