21217865 has 4 divisors (see below), whose sum is σ = 25461444. Its totient is φ = 16974288.

The previous prime is 21217849. The next prime is 21217907. The reversal of 21217865 is 56871212.

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 2650384 + 18567481 = 1628^2 + 4309^2 .

It is a cyclic number.

It is not a de Polignac number, because 21217865 - 2⁴ = 21217849 is a prime.

It is a Duffinian number.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2121782 + ... + 2121791.

It is an arithmetic number, because the mean of its divisors is an integer number (6365361).

Almost surely, 2^21217865 is an apocalyptic number.

It is an amenable number.

21217865 is a deficient number, since it is larger than the sum of its proper divisors (4243579).

21217865 is an equidigital number, since it uses as much as digits as its factorization.

21217865 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4243578.

The product of its digits is 6720, while the sum is 32.

The square root of 21217865 is about 4606.2853797827. The cubic root of 21217865 is about 276.8432212863.

The spelling of 21217865 in words is "twenty-one million, two hundred seventeen thousand, eight hundred sixty-five".