21365813 has 16 divisors (see below), whose sum is σ = 25132800. Its totient is φ = 18155088.

The previous prime is 21365809. The next prime is 21365819. The reversal of 21365813 is 31856312.

It is not a de Polignac number, because 21365813 - 2² = 21365809 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21365819) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 57095 + ... + 57467.

It is an arithmetic number, because the mean of its divisors is an integer number (1570800).

Almost surely, 2^21365813 is an apocalyptic number.

It is an amenable number.

21365813 is a deficient number, since it is larger than the sum of its proper divisors (3766987).

21365813 is an equidigital number, since it uses as much as digits as its factorization.

21365813 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 561 (or 547 counting only the distinct ones).

The product of its digits is 4320, while the sum is 29.

The square root of 21365813 is about 4622.3168433157. The cubic root of 21365813 is about 277.4851892645.

The spelling of 21365813 in words is "twenty-one million, three hundred sixty-five thousand, eight hundred thirteen".