21644765 has 4 divisors (see below), whose sum is σ = 25973724. Its totient is φ = 17315808.

The previous prime is 21644747. The next prime is 21644801. The reversal of 21644765 is 56744612.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 3171961 + 18472804 = 1781^2 + 4298^2 .

It is a cyclic number.

It is not a de Polignac number, because 21644765 - 2⁸ = 21644509 is a prime.

It is a Duffinian number.

It is a congruent number.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2164472 + ... + 2164481.

It is an arithmetic number, because the mean of its divisors is an integer number (6493431).

Almost surely, 2^21644765 is an apocalyptic number.

It is an amenable number.

21644765 is a deficient number, since it is larger than the sum of its proper divisors (4328959).

21644765 is an equidigital number, since it uses as much as digits as its factorization.

21644765 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4328958.

The product of its digits is 40320, while the sum is 35.

The square root of 21644765 is about 4652.3934700324. The cubic root of 21644765 is about 278.6875868228.

The spelling of 21644765 in words is "twenty-one million, six hundred forty-four thousand, seven hundred sixty-five".