21091809 has 4 divisors (see below), whose sum is σ = 28122416. Its totient is φ = 14061204.

The previous prime is 21091793. The next prime is 21091823. The reversal of 21091809 is 90819012.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is not a de Polignac number, because 21091809 - 2⁴ = 21091793 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 (21091009) 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, 3515299 + ... + 3515304.

It is an arithmetic number, because the mean of its divisors is an integer number (7030604).

Almost surely, 2^21091809 is an apocalyptic number.

It is an amenable number.

21091809 is a deficient number, since it is larger than the sum of its proper divisors (7030607).

21091809 is an equidigital number, since it uses as much as digits as its factorization.

21091809 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7030606.

The product of its (nonzero) digits is 1296, while the sum is 30.

The square root of 21091809 is about 4592.5819535420. The cubic root of 21091809 is about 276.2938872283.

The spelling of 21091809 in words is "twenty-one million, ninety-one thousand, eight hundred nine".