21507809 has 2 divisors, whose sum is σ = 21507810. Its totient is φ = 21507808.

The previous prime is 21507803. The next prime is 21507833. The reversal of 21507809 is 90870512.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 13667809 + 7840000 = 3697^2 + 2800^2 .

It is a cyclic number.

It is not a de Polignac number, because 21507809 - 2⁴ = 21507793 is a prime.

It is a Sophie Germain prime.

It is a Curzon number.

It is not a weakly prime, because it can be changed into another prime (21507803) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10753904 + 10753905.

It is an arithmetic number, because the mean of its divisors is an integer number (10753905).

Almost surely, 2^21507809 is an apocalyptic number.

It is an amenable number.

21507809 is a deficient number, since it is larger than the sum of its proper divisors (1).

21507809 is an equidigital number, since it uses as much as digits as its factorization.

21507809 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 5040, while the sum is 32.

The square root of 21507809 is about 4637.6512374261. The cubic root of 21507809 is about 278.0985495024.

The spelling of 21507809 in words is "twenty-one million, five hundred seven thousand, eight hundred nine".