21125813 has 4 divisors (see below), whose sum is σ = 21138828. Its totient is φ = 21112800.

The previous prime is 21125803. The next prime is 21125843. The reversal of 21125813 is 31852112.

It is a happy number.

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 677329 + 20448484 = 823^2 + 4522^2 .

It is a cyclic number.

It is not a de Polignac number, because 21125813 - 2⁶ = 21125749 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21125803) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3656 + ... + 7457.

It is an arithmetic number, because the mean of its divisors is an integer number (5284707).

Almost surely, 2^21125813 is an apocalyptic number.

It is an amenable number.

21125813 is a deficient number, since it is larger than the sum of its proper divisors (13015).

21125813 is a wasteful number, since it uses less digits than its factorization.

21125813 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 13014.

The product of its digits is 480, while the sum is 23.

The square root of 21125813 is about 4596.2825196021. The cubic root of 21125813 is about 276.4422869058.

Adding to 21125813 its reverse (31852112), we get a palindrome (52977925).

The spelling of 21125813 in words is "twenty-one million, one hundred twenty-five thousand, eight hundred thirteen".