21561865 has 48 divisors (see below), whose sum is σ = 31622400. Its totient is φ = 14017536.

The previous prime is 21561851. The next prime is 21561887. The reversal of 21561865 is 56816512.

It is a happy number.

It is not a de Polignac number, because 21561865 - 2⁷ = 21561737 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21561865.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 47 ways as a sum of consecutive naturals, for example, 272896 + ... + 272974.

It is an arithmetic number, because the mean of its divisors is an integer number (658800).

Almost surely, 2^21561865 is an apocalyptic number.

It is an amenable number.

21561865 is a deficient number, since it is larger than the sum of its proper divisors (10060535).

21561865 is a wasteful number, since it uses less digits than its factorization.

21561865 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 146 (or 133 counting only the distinct ones).

The product of its digits is 14400, while the sum is 34.

The square root of 21561865 is about 4643.4755302467. The cubic root of 21561865 is about 278.3313381079.

The spelling of 21561865 in words is "twenty-one million, five hundred sixty-one thousand, eight hundred sixty-five".