21387531 has 8 divisors (see below), whose sum is σ = 31109184. Its totient is φ = 12962120.

The previous prime is 21387511. The next prime is 21387557. The reversal of 21387531 is 13578312.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21387531 - 2⁷ = 21387403 is a prime.

It is a super-2 number, since 2×21387531² = 914852964551922, which contains 22 as substring.

It is not an unprimeable number, because it can be changed into a prime (21387511) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 324021 + ... + 324086.

It is an arithmetic number, because the mean of its divisors is an integer number (3888648).

Almost surely, 2^21387531 is an apocalyptic number.

21387531 is a deficient number, since it is larger than the sum of its proper divisors (9721653).

21387531 is a wasteful number, since it uses less digits than its factorization.

21387531 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 648121.

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

The square root of 21387531 is about 4624.6655014174. The cubic root of 21387531 is about 277.5791771471.

It can be divided in two parts, 2138 and 7531, that added together give a palindrome (9669).

The spelling of 21387531 in words is "twenty-one million, three hundred eighty-seven thousand, five hundred thirty-one".