21972391 has 4 divisors (see below), whose sum is σ = 25111312. Its totient is φ = 18833472.

The previous prime is 21972373. The next prime is 21972407. The reversal of 21972391 is 19327912.

It is a happy number.

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

It is not a de Polignac number, because 21972391 - 2¹⁷ = 21841319 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21972331) 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, 1569450 + ... + 1569463.

It is an arithmetic number, because the mean of its divisors is an integer number (6277828).

Almost surely, 2^21972391 is an apocalyptic number.

21972391 is a deficient number, since it is larger than the sum of its proper divisors (3138921).

21972391 is an equidigital number, since it uses as much as digits as its factorization.

21972391 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3138920.

The product of its digits is 6804, while the sum is 34.

The square root of 21972391 is about 4687.4717065813. The cubic root of 21972391 is about 280.0866695986.

The spelling of 21972391 in words is "twenty-one million, nine hundred seventy-two thousand, three hundred ninety-one".