21991257 has 12 divisors (see below), whose sum is σ = 32942000. Its totient is φ = 14660676.

The previous prime is 21991231. The next prime is 21991267. The reversal of 21991257 is 75219912.

It is not a de Polignac number, because 21991257 - 2⁸ = 21991001 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21991267) 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, 45007 + ... + 45492.

Almost surely, 2^21991257 is an apocalyptic number.

21991257 is a gapful number since it is divisible by the number (27) formed by its first and last digit.

It is an amenable number.

21991257 is a deficient number, since it is larger than the sum of its proper divisors (10950743).

21991257 is an frugal number, since it uses more digits than its factorization.

21991257 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 90514 (or 90502 counting only the distinct ones).

The product of its digits is 11340, while the sum is 36.

The square root of 21991257 is about 4689.4836602765. The cubic root of 21991257 is about 280.1668096173.

The spelling of 21991257 in words is "twenty-one million, nine hundred ninety-one thousand, two hundred fifty-seven".