21685509 has 16 divisors (see below), whose sum is σ = 32673600. Its totient is φ = 14210928.

The previous prime is 21685507. The next prime is 21685513. The reversal of 21685509 is 90558612.

It is a happy number.

It is not a de Polignac number, because 21685509 - 2¹ = 21685507 is a prime.

It is a super-2 number, since 2×21685509² = 940522601178162, which contains 22 as substring.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21685507) 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 in 15 ways as a sum of consecutive naturals, for example, 5214 + ... + 8399.

It is an arithmetic number, because the mean of its divisors is an integer number (2042100).

Almost surely, 2^21685509 is an apocalyptic number.

It is an amenable number.

21685509 is a deficient number, since it is larger than the sum of its proper divisors (10988091).

21685509 is a wasteful number, since it uses less digits than its factorization.

21685509 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 13681 (or 13675 counting only the distinct ones).

The product of its (nonzero) digits is 21600, while the sum is 36.

The square root of 21685509 is about 4656.7702326827. The cubic root of 21685509 is about 278.8623439324.

The spelling of 21685509 in words is "twenty-one million, six hundred eighty-five thousand, five hundred nine".