21538613 has 4 divisors (see below), whose sum is σ = 21547980. Its totient is φ = 21529248.

The previous prime is 21538597. The next prime is 21538651. The reversal of 21538613 is 31683512.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 3713329 + 17825284 = 1927^2 + 4222^2 .

It is a cyclic number.

It is not a de Polignac number, because 21538613 - 2⁴ = 21538597 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 (21532613) 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, 1403 + ... + 6711.

It is an arithmetic number, because the mean of its divisors is an integer number (5386995).

Almost surely, 2^21538613 is an apocalyptic number.

It is an amenable number.

21538613 is a deficient number, since it is larger than the sum of its proper divisors (9367).

21538613 is an equidigital number, since it uses as much as digits as its factorization.

21538613 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 9366.

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

The square root of 21538613 is about 4640.9711268225. The cubic root of 21538613 is about 278.2312526403.

The spelling of 21538613 in words is "twenty-one million, five hundred thirty-eight thousand, six hundred thirteen".