21936209 has 4 divisors (see below), whose sum is σ = 22692660. Its totient is φ = 21179760.

The previous prime is 21936191. The next prime is 21936217. The reversal of 21936209 is 90263912.

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 4060225 + 17875984 = 2015^2 + 4228^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21936209 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21933209) 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, 378182 + ... + 378239.

It is an arithmetic number, because the mean of its divisors is an integer number (5673165).

Almost surely, 2^21936209 is an apocalyptic number.

21936209 is a gapful number since it is divisible by the number (29) formed by its first and last digit.

It is an amenable number.

21936209 is a deficient number, since it is larger than the sum of its proper divisors (756451).

21936209 is an equidigital number, since it uses as much as digits as its factorization.

21936209 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 756450.

The product of its (nonzero) digits is 5832, while the sum is 32.

The square root of 21936209 is about 4683.6106798068. The cubic root of 21936209 is about 279.9328452896.

The spelling of 21936209 in words is "twenty-one million, nine hundred thirty-six thousand, two hundred nine".