21639931 has 4 divisors (see below), whose sum is σ = 22224832. Its totient is φ = 21055032.

The previous prime is 21639889. The next prime is 21639967. The reversal of 21639931 is 13993612.

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 21639931 - 2¹³ = 21631739 is a prime.

It is a super-2 number, since 2×21639931² = 936573227369522, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21639731) 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, 292395 + ... + 292468.

It is an arithmetic number, because the mean of its divisors is an integer number (5556208).

Almost surely, 2^21639931 is an apocalyptic number.

21639931 is a deficient number, since it is larger than the sum of its proper divisors (584901).

21639931 is an equidigital number, since it uses as much as digits as its factorization.

21639931 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 584900.

The product of its digits is 8748, while the sum is 34.

The square root of 21639931 is about 4651.8739234850. The cubic root of 21639931 is about 278.6668385249. Note that the first 3 decimals are identical.

The spelling of 21639931 in words is "twenty-one million, six hundred thirty-nine thousand, nine hundred thirty-one".