21856409 has 2 divisors, whose sum is σ = 21856410. Its totient is φ = 21856408.

The previous prime is 21856399. The next prime is 21856423. The reversal of 21856409 is 90465812.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 21855625 + 784 = 4675^2 + 28^2 .

It is a cyclic number.

It is not a de Polignac number, because 21856409 - 2²⁰ = 20807833 is a prime.

It is a super-2 number, since 2×21856409² = 955405228750562, which contains 22 as substring.

It is a Sophie Germain prime.

It is a Chen prime.

It is a Curzon number.

It is not a weakly prime, because it can be changed into another prime (21856469) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10928204 + 10928205.

It is an arithmetic number, because the mean of its divisors is an integer number (10928205).

Almost surely, 2^21856409 is an apocalyptic number.

It is an amenable number.

21856409 is a deficient number, since it is larger than the sum of its proper divisors (1).

21856409 is an equidigital number, since it uses as much as digits as its factorization.

21856409 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 17280, while the sum is 35.

The square root of 21856409 is about 4675.0838495154. The cubic root of 21856409 is about 279.5929843178.

The spelling of 21856409 in words is "twenty-one million, eight hundred fifty-six thousand, four hundred nine".