20856809 has 4 divisors (see below), whose sum is σ = 20882004. Its totient is φ = 20831616.

The previous prime is 20856793. The next prime is 20856827. The reversal of 20856809 is 90865802.

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 20430400 + 426409 = 4520^2 + 653^2 .

It is a cyclic number.

It is not a de Polignac number, because 20856809 - 2⁴ = 20856793 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (20856839) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 11312 + ... + 13025.

It is an arithmetic number, because the mean of its divisors is an integer number (5220501).

Almost surely, 2^20856809 is an apocalyptic number.

It is an amenable number.

20856809 is a deficient number, since it is larger than the sum of its proper divisors (25195).

20856809 is an equidigital number, since it uses as much as digits as its factorization.

20856809 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 25194.

The product of its (nonzero) digits is 34560, while the sum is 38.

The square root of 20856809 is about 4566.9255522726. The cubic root of 20856809 is about 275.2639185362.

The spelling of 20856809 in words is "twenty million, eight hundred fifty-six thousand, eight hundred nine".