21991881 has 4 divisors (see below), whose sum is σ = 29322512. Its totient is φ = 14661252.

The previous prime is 21991847. The next prime is 21991883. The reversal of 21991881 is 18819912.

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 21991881 - 2¹¹ = 21989833 is a prime.

It is a super-2 number, since 2×21991881² = 967285659836322, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21991883) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3665311 + ... + 3665316.

It is an arithmetic number, because the mean of its divisors is an integer number (7330628).

Almost surely, 2^21991881 is an apocalyptic number.

It is an amenable number.

21991881 is a deficient number, since it is larger than the sum of its proper divisors (7330631).

21991881 is an equidigital number, since it uses as much as digits as its factorization.

21991881 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7330630.

The product of its digits is 10368, while the sum is 39.

The square root of 21991881 is about 4689.5501916495. The cubic root of 21991881 is about 280.1694594951.

The spelling of 21991881 in words is "twenty-one million, nine hundred ninety-one thousand, eight hundred eighty-one".