21138809 has 4 divisors (see below), whose sum is σ = 21255780. Its totient is φ = 21021840.

The previous prime is 21138791. The next prime is 21138823. The reversal of 21138809 is 90883112.

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 162409 + 20976400 = 403^2 + 4580^2 .

It is a cyclic number.

It is not a de Polignac number, because 21138809 - 2¹² = 21134713 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21138209) 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, 58214 + ... + 58575.

It is an arithmetic number, because the mean of its divisors is an integer number (5313945).

Almost surely, 2^21138809 is an apocalyptic number.

It is an amenable number.

21138809 is a deficient number, since it is larger than the sum of its proper divisors (116971).

21138809 is a wasteful number, since it uses less digits than its factorization.

21138809 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 116970.

The product of its (nonzero) digits is 3456, while the sum is 32.

The square root of 21138809 is about 4597.6960534598. The cubic root of 21138809 is about 276.4989617664.

The spelling of 21138809 in words is "twenty-one million, one hundred thirty-eight thousand, eight hundred nine".