21156809 has 2 divisors, whose sum is σ = 21156810. Its totient is φ = 21156808.

The previous prime is 21156769. The next prime is 21156829. The reversal of 21156809 is 90865112.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 18361225 + 2795584 = 4285^2 + 1672^2 .

It is a cyclic number.

It is not a de Polignac number, because 21156809 - 2¹² = 21152713 is a prime.

It is a super-2 number, since 2×21156809² = 895221134124962, which contains 22 as substring.

It is not a weakly prime, because it can be changed into another prime (21156829) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10578404 + 10578405.

It is an arithmetic number, because the mean of its divisors is an integer number (10578405).

Almost surely, 2^21156809 is an apocalyptic number.

It is an amenable number.

21156809 is a deficient number, since it is larger than the sum of its proper divisors (1).

21156809 is an equidigital number, since it uses as much as digits as its factorization.

21156809 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 4320, while the sum is 32.

The square root of 21156809 is about 4599.6531390965. The cubic root of 21156809 is about 276.5774204490.

The spelling of 21156809 in words is "twenty-one million, one hundred fifty-six thousand, eight hundred nine".