21013865 has 4 divisors (see below), whose sum is σ = 25216644. Its totient is φ = 16811088.

The previous prime is 21013859. The next prime is 21013871. The reversal of 21013865 is 56831012.

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

It is an interprime number because it is at equal distance from previous prime (21013859) and next prime (21013871).

It can be written as a sum of positive squares in 2 ways, for example, as 12931216 + 8082649 = 3596^2 + 2843^2 .

It is a cyclic number.

It is not a de Polignac number, because 21013865 - 2⁴ = 21013849 is a prime.

It is a Duffinian number.

It is a Curzon 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, 2101382 + ... + 2101391.

It is an arithmetic number, because the mean of its divisors is an integer number (6304161).

Almost surely, 2^21013865 is an apocalyptic number.

It is an amenable number.

21013865 is a deficient number, since it is larger than the sum of its proper divisors (4202779).

21013865 is an equidigital number, since it uses as much as digits as its factorization.

21013865 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4202778.

The product of its (nonzero) digits is 1440, while the sum is 26.

The square root of 21013865 is about 4584.0882408610. The cubic root of 21013865 is about 275.9531225083.

Adding to 21013865 its reverse (56831012), we get a palindrome (77844877).

The spelling of 21013865 in words is "twenty-one million, thirteen thousand, eight hundred sixty-five".