21565891 has 2 divisors, whose sum is σ = 21565892. Its totient is φ = 21565890.

The previous prime is 21565867. The next prime is 21565897. The reversal of 21565891 is 19856512.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21565891 - 2¹⁵ = 21533123 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21565891.

It is not a weakly prime, because it can be changed into another prime (21565897) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10782945 + 10782946.

It is an arithmetic number, because the mean of its divisors is an integer number (10782946).

Almost surely, 2^21565891 is an apocalyptic number.

21565891 is a deficient number, since it is larger than the sum of its proper divisors (1).

21565891 is an equidigital number, since it uses as much as digits as its factorization.

21565891 is an evil number, because the sum of its binary digits is even.

The product of its digits is 21600, while the sum is 37.

The square root of 21565891 is about 4643.9090215033. The cubic root of 21565891 is about 278.3486602370.

The spelling of 21565891 in words is "twenty-one million, five hundred sixty-five thousand, eight hundred ninety-one".