21201809 has 4 divisors (see below), whose sum is σ = 21354480. Its totient is φ = 21049140.

The previous prime is 21201787. The next prime is 21201811. The reversal of 21201809 is 90810212.

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 a cyclic number.

It is not a de Polignac number, because 21201809 - 2⁸ = 21201553 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21201889) 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, 76127 + ... + 76404.

It is an arithmetic number, because the mean of its divisors is an integer number (5338620).

Almost surely, 2^21201809 is an apocalyptic number.

It is an amenable number.

21201809 is a deficient number, since it is larger than the sum of its proper divisors (152671).

21201809 is a wasteful number, since it uses less digits than its factorization.

21201809 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 152670.

The product of its (nonzero) digits is 288, while the sum is 23.

The square root of 21201809 is about 4604.5422139448. The cubic root of 21201809 is about 276.7733726558.

The spelling of 21201809 in words is "twenty-one million, two hundred one thousand, eight hundred nine".