21979391 has 12 divisors (see below), whose sum is σ = 25644528. Its totient is φ = 18783072.

The previous prime is 21979387. The next prime is 21979423. The reversal of 21979391 is 19397912.

It is not a de Polignac number, because 21979391 - 2² = 21979387 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21979091) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 29870 + ... + 30596.

It is an arithmetic number, because the mean of its divisors is an integer number (2137044).

Almost surely, 2^21979391 is an apocalyptic number.

21979391 is a deficient number, since it is larger than the sum of its proper divisors (3665137).

21979391 is an equidigital number, since it uses as much as digits as its factorization.

21979391 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1358 (or 1351 counting only the distinct ones).

The product of its digits is 30618, while the sum is 41.

The square root of 21979391 is about 4688.2183182953. The cubic root of 21979391 is about 280.1164099292.

The spelling of 21979391 in words is "twenty-one million, nine hundred seventy-nine thousand, three hundred ninety-one".