21371809 has 2 divisors, whose sum is σ = 21371810. Its totient is φ = 21371808.

The previous prime is 21371797. The next prime is 21371821. The reversal of 21371809 is 90817312.

It is a balanced prime because it is at equal distance from previous prime (21371797) and next prime (21371821).

It can be written as a sum of positive squares in only one way, i.e., 16974400 + 4397409 = 4120^2 + 2097^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21371809 is a prime.

It is not a weakly prime, because it can be changed into another prime (21371849) 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, 10685904 + 10685905.

It is an arithmetic number, because the mean of its divisors is an integer number (10685905).

Almost surely, 2^21371809 is an apocalyptic number.

It is an amenable number.

21371809 is a deficient number, since it is larger than the sum of its proper divisors (1).

21371809 is an equidigital number, since it uses as much as digits as its factorization.

21371809 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 3024, while the sum is 31.

The square root of 21371809 is about 4622.9653903096. The cubic root of 21371809 is about 277.5111442106.

The spelling of 21371809 in words is "twenty-one million, three hundred seventy-one thousand, eight hundred nine".