21991572 has 18 divisors (see below), whose sum is σ = 55589898. Its totient is φ = 7330512.

The previous prime is 21991561. The next prime is 21991573. The reversal of 21991572 is 27519912.

It can be written as a sum of positive squares in only one way, i.e., 11532816 + 10458756 = 3396^2 + 3234^2 .

It is a tau number, because it is divible by the number of its divisors (18).

It is a Harshad number since it is a multiple of its sum of digits (36), and also a Moran number because the ratio is a prime number: 610877 = 21991572 / (2 + 1 + 9 + 9 + 1 + 5 + 7 + 2).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21991573) by changing a digit.

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 5 ways as a sum of consecutive naturals, for example, 305403 + ... + 305474.

Almost surely, 2^21991572 is an apocalyptic number.

It is an amenable number.

21991572 is an abundant number, since it is smaller than the sum of its proper divisors (33598326).

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

21991572 is a wasteful number, since it uses less digits than its factorization.

21991572 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 610887 (or 610882 counting only the distinct ones).

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

The square root of 21991572 is about 4689.5172459433. The cubic root of 21991572 is about 280.1681473023.

The spelling of 21991572 in words is "twenty-one million, nine hundred ninety-one thousand, five hundred seventy-two".