21787991 has 4 divisors (see below), whose sum is σ = 21814800. Its totient is φ = 21761184.

The previous prime is 21787981. The next prime is 21788017. The reversal of 21787991 is 19978712.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21787991 - 2¹⁰ = 21786967 is a prime.

It is a Duffinian number.

It is a zygodrome in base 3.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21787981) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 12146 + ... + 13823.

It is an arithmetic number, because the mean of its divisors is an integer number (5453700).

Almost surely, 2^21787991 is an apocalyptic number.

21787991 is a deficient number, since it is larger than the sum of its proper divisors (26809).

21787991 is an equidigital number, since it uses as much as digits as its factorization.

21787991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 26808.

The product of its digits is 63504, while the sum is 44.

The square root of 21787991 is about 4667.7608122096. The cubic root of 21787991 is about 279.3009389673.

The spelling of 21787991 in words is "twenty-one million, seven hundred eighty-seven thousand, nine hundred ninety-one".