21931257 has 4 divisors (see below), whose sum is σ = 29241680. Its totient is φ = 14620836.

The previous prime is 21931253. The next prime is 21931271. The reversal of 21931257 is 75213912.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is not a de Polignac number, because 21931257 - 2² = 21931253 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21931253) 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, 3655207 + ... + 3655212.

It is an arithmetic number, because the mean of its divisors is an integer number (7310420).

Almost surely, 2^21931257 is an apocalyptic number.

It is an amenable number.

21931257 is a deficient number, since it is larger than the sum of its proper divisors (7310423).

21931257 is an equidigital number, since it uses as much as digits as its factorization.

21931257 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7310422.

The product of its digits is 3780, while the sum is 30.

The square root of 21931257 is about 4683.0819980009. The cubic root of 21931257 is about 279.9117791796.

The spelling of 21931257 in words is "twenty-one million, nine hundred thirty-one thousand, two hundred fifty-seven".