21565942581 has 16 divisors (see below), whose sum is σ = 30088952064. Its totient is φ = 13724777664.

The previous prime is 21565942561. The next prime is 21565942633. The reversal of 21565942581 is 18524956512.

It is not a de Polignac number, because 21565942581 - 2²³ = 21557553973 is a prime.

It is a super-2 number, since 2×21565942581² (a number of 21 digits) contains 22 as substring.

It is a Curzon number.

It is a self number, because there is not a number n which added to its sum of digits gives 21565942581.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21565942561) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 1827021 + ... + 1838786.

It is an arithmetic number, because the mean of its divisors is an integer number (1880559504).

Almost surely, 2^21565942581 is an apocalyptic number.

It is an amenable number.

21565942581 is a deficient number, since it is larger than the sum of its proper divisors (8523009483).

21565942581 is a wasteful number, since it uses less digits than its factorization.

21565942581 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3665900.

The product of its digits is 864000, while the sum is 48.

The spelling of 21565942581 in words is "twenty-one billion, five hundred sixty-five million, nine hundred forty-two thousand, five hundred eighty-one".