21565591 has 2 divisors, whose sum is σ = 21565592. Its totient is φ = 21565590.

The previous prime is 21565573. The next prime is 21565597. The reversal of 21565591 is 19556512.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21565591 - 2⁷ = 21565463 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21565597) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10782795 + 10782796.

It is an arithmetic number, because the mean of its divisors is an integer number (10782796).

Almost surely, 2^21565591 is an apocalyptic number.

21565591 is a deficient number, since it is larger than the sum of its proper divisors (1).

21565591 is an equidigital number, since it uses as much as digits as its factorization.

21565591 is an evil number, because the sum of its binary digits is even.

The product of its digits is 13500, while the sum is 34.

The square root of 21565591 is about 4643.8767210166. The cubic root of 21565591 is about 278.3473695417.

The spelling of 21565591 in words is "twenty-one million, five hundred sixty-five thousand, five hundred ninety-one".