21040865 has 4 divisors (see below), whose sum is σ = 25249044. Its totient is φ = 16832688.

The previous prime is 21040807. The next prime is 21040879. The reversal of 21040865 is 56804012.

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 9778129 + 11262736 = 3127^2 + 3356^2 .

It is a cyclic number.

It is not a de Polignac number, because 21040865 - 2¹⁸ = 20778721 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, 2104082 + ... + 2104091.

It is an arithmetic number, because the mean of its divisors is an integer number (6312261).

Almost surely, 2^21040865 is an apocalyptic number.

It is an amenable number.

21040865 is a deficient number, since it is larger than the sum of its proper divisors (4208179).

21040865 is an equidigital number, since it uses as much as digits as its factorization.

21040865 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4208178.

The product of its (nonzero) digits is 1920, while the sum is 26.

The square root of 21040865 is about 4587.0322649835. The cubic root of 21040865 is about 276.0712595180.

Adding to 21040865 its reverse (56804012), we get a palindrome (77844877).

The spelling of 21040865 in words is "twenty-one million, forty thousand, eight hundred sixty-five".