21091209 has 4 divisors (see below), whose sum is σ = 28121616. Its totient is φ = 14060804.

The previous prime is 21091201. The next prime is 21091249. The reversal of 21091209 is 90219012.

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 21091209 - 2³ = 21091201 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21091201) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3515199 + ... + 3515204.

It is an arithmetic number, because the mean of its divisors is an integer number (7030404).

Almost surely, 2^21091209 is an apocalyptic number.

It is an amenable number.

21091209 is a deficient number, since it is larger than the sum of its proper divisors (7030407).

21091209 is an equidigital number, since it uses as much as digits as its factorization.

21091209 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7030406.

The product of its (nonzero) digits is 324, while the sum is 24.

The square root of 21091209 is about 4592.5166303455. The cubic root of 21091209 is about 276.2912672870.

It can be divided in two parts, 210 and 91209, that added together give a palindrome (91419).

The spelling of 21091209 in words is "twenty-one million, ninety-one thousand, two hundred nine".