21991720 has 32 divisors (see below), whose sum is σ = 50298120. Its totient is φ = 8651520.

The previous prime is 21991699. The next prime is 21991733. The reversal of 21991720 is 2719912.

It can be written as a sum of positive squares in 4 ways, for example, as 11009124 + 10982596 = 3318^2 + 3314^2 .

It is a Smith number, since the sum of its digits (31) coincides with the sum of the digits of its prime factors.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 2067 + ... + 6946.

Almost surely, 2^21991720 is an apocalyptic number.

21991720 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21991720 is an abundant number, since it is smaller than the sum of its proper divisors (28306400).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21991720 is a wasteful number, since it uses less digits than its factorization.

21991720 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9085 (or 9081 counting only the distinct ones).

The product of its (nonzero) digits is 2268, while the sum is 31.

The square root of 21991720 is about 4689.5330257927. The cubic root of 21991720 is about 280.1687757975.

The spelling of 21991720 in words is "twenty-one million, nine hundred ninety-one thousand, seven hundred twenty".