21908609 has 2 divisors, whose sum is σ = 21908610. Its totient is φ = 21908608.

The previous prime is 21908603. The next prime is 21908611. The reversal of 21908609 is 90680912.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 18835600 + 3073009 = 4340^2 + 1753^2 .

It is a cyclic number.

It is not a de Polignac number, because 21908609 - 2²⁰ = 20860033 is a prime.

Together with 21908611, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21908603) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10954304 + 10954305.

It is an arithmetic number, because the mean of its divisors is an integer number (10954305).

Almost surely, 2^21908609 is an apocalyptic number.

It is an amenable number.

21908609 is a deficient number, since it is larger than the sum of its proper divisors (1).

21908609 is an equidigital number, since it uses as much as digits as its factorization.

21908609 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 7776, while the sum is 35.

The square root of 21908609 is about 4680.6633076948. The cubic root of 21908609 is about 279.8153927689.

The spelling of 21908609 in words is "twenty-one million, nine hundred eight thousand, six hundred nine".