21644865 has 48 divisors (see below), whose sum is σ = 41558400. Its totient is φ = 10333440.

The previous prime is 21644849. The next prime is 21644879. The reversal of 21644865 is 56844612.

21644865 is a `hidden beast` number, since 2 + 1 + 644 + 8 + 6 + 5 = 666.

It is not a de Polignac number, because 21644865 - 2⁴ = 21644849 is a prime.

It is a Curzon number.

It is an unprimeable number.

It is a polite number, since it can be written in 47 ways as a sum of consecutive naturals, for example, 35836 + ... + 36434.

It is an arithmetic number, because the mean of its divisors is an integer number (865800).

Almost surely, 2^21644865 is an apocalyptic number.

It is an amenable number.

21644865 is a deficient number, since it is larger than the sum of its proper divisors (19913535).

21644865 is a wasteful number, since it uses less digits than its factorization.

21644865 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 694 (or 691 counting only the distinct ones).

The product of its digits is 46080, while the sum is 36.

The square root of 21644865 is about 4652.4042171763. The cubic root of 21644865 is about 278.6880160061.

The spelling of 21644865 in words is "twenty-one million, six hundred forty-four thousand, eight hundred sixty-five".