21991311 has 8 divisors (see below), whose sum is σ = 32579760. Its totient is φ = 14660856.

The previous prime is 21991301. The next prime is 21991313. The reversal of 21991311 is 11319912.

It is not a de Polignac number, because 21991311 - 2⁵ = 21991279 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27), and also a Moran number because the ratio is a prime number: 814493 = 21991311 / (2 + 1 + 9 + 9 + 1 + 3 + 1 + 1).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21991313) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 407220 + ... + 407273.

It is an arithmetic number, because the mean of its divisors is an integer number (4072470).

Almost surely, 2^21991311 is an apocalyptic number.

21991311 is a deficient number, since it is larger than the sum of its proper divisors (10588449).

21991311 is an equidigital number, since it uses as much as digits as its factorization.

21991311 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 814502 (or 814496 counting only the distinct ones).

The product of its digits is 486, while the sum is 27.

The square root of 21991311 is about 4689.4894178364. The cubic root of 21991311 is about 280.1670389356.

The spelling of 21991311 in words is "twenty-one million, nine hundred ninety-one thousand, three hundred eleven".