21656909 has 2 divisors, whose sum is σ = 21656910. Its totient is φ = 21656908.

The previous prime is 21656893. The next prime is 21656911. The reversal of 21656909 is 90965612.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 19758025 + 1898884 = 4445^2 + 1378^2 .

It is a cyclic number.

It is not a de Polignac number, because 21656909 - 2⁴ = 21656893 is a prime.

It is a Sophie Germain prime.

Together with 21656911, it forms a pair of twin primes.

It is a Chen prime.

It is an alternating number because its digits alternate between even and odd.

It is a Curzon number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21656969) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10828454 + 10828455.

It is an arithmetic number, because the mean of its divisors is an integer number (10828455).

Almost surely, 2^21656909 is an apocalyptic number.

It is an amenable number.

21656909 is a deficient number, since it is larger than the sum of its proper divisors (1).

21656909 is an equidigital number, since it uses as much as digits as its factorization.

21656909 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 29160, while the sum is 38.

The square root of 21656909 is about 4653.6984216857. The cubic root of 21656909 is about 278.7396971800.

The spelling of 21656909 in words is "twenty-one million, six hundred fifty-six thousand, nine hundred nine".