21685309 has 2 divisors, whose sum is σ = 21685310. Its totient is φ = 21685308.

The previous prime is 21685277. The next prime is 21685333. The reversal of 21685309 is 90358612.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 18896409 + 2788900 = 4347^2 + 1670^2 .

It is a cyclic number.

It is not a de Polignac number, because 21685309 - 2⁵ = 21685277 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21685379) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10842654 + 10842655.

It is an arithmetic number, because the mean of its divisors is an integer number (10842655).

Almost surely, 2^21685309 is an apocalyptic number.

It is an amenable number.

21685309 is a deficient number, since it is larger than the sum of its proper divisors (1).

21685309 is an equidigital number, since it uses as much as digits as its factorization.

21685309 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 12960, while the sum is 34.

The square root of 21685309 is about 4656.7487585224. The cubic root of 21685309 is about 278.8614866374.

The spelling of 21685309 in words is "twenty-one million, six hundred eighty-five thousand, three hundred nine".