21387631 has 4 divisors (see below), whose sum is σ = 22317552. Its totient is φ = 20457712.

The previous prime is 21387629. The next prime is 21387643. The reversal of 21387631 is 13678312.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21387631 - 2¹ = 21387629 is a prime.

It is a super-2 number, since 2×21387631² = 914861519584322, which contains 22 as substring.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21387593 and 21387602.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21387601) 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, 464926 + ... + 464971.

It is an arithmetic number, because the mean of its divisors is an integer number (5579388).

Almost surely, 2^21387631 is an apocalyptic number.

21387631 is a deficient number, since it is larger than the sum of its proper divisors (929921).

21387631 is an equidigital number, since it uses as much as digits as its factorization.

21387631 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 929920.

The product of its digits is 6048, while the sum is 31.

The square root of 21387631 is about 4624.6763129975. The cubic root of 21387631 is about 277.5796097648.

The spelling of 21387631 in words is "twenty-one million, three hundred eighty-seven thousand, six hundred thirty-one".