21991209 has 16 divisors (see below), whose sum is σ = 30573888. Its totient is φ = 14046912.

The previous prime is 21991199. The next prime is 21991219. The reversal of 21991209 is 90219912.

It is an interprime number because it is at equal distance from previous prime (21991199) and next prime (21991219).

It is not a de Polignac number, because 21991209 - 2⁹ = 21990697 is a prime.

It is a super-2 number, since 2×21991209² = 967226546563362, which contains 22 as substring.

It is a self number, because there is not a number n which added to its sum of digits gives 21991209.

It is not an unprimeable number, because it can be changed into a prime (21991219) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 5959 + ... + 8915.

It is an arithmetic number, because the mean of its divisors is an integer number (1910868).

Almost surely, 2^21991209 is an apocalyptic number.

It is an amenable number.

21991209 is a deficient number, since it is larger than the sum of its proper divisors (8582679).

21991209 is a wasteful number, since it uses less digits than its factorization.

21991209 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3064.

The product of its (nonzero) digits is 2916, while the sum is 33.

The square root of 21991209 is about 4689.4785424394. The cubic root of 21991209 is about 280.1666057784.

The spelling of 21991209 in words is "twenty-one million, nine hundred ninety-one thousand, two hundred nine".