21685687 has 2 divisors, whose sum is σ = 21685688. Its totient is φ = 21685686.

The previous prime is 21685667. The next prime is 21685721. The reversal of 21685687 is 78658612.

It is a weak prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21685687 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21685607) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10842843 + 10842844.

It is an arithmetic number, because the mean of its divisors is an integer number (10842844).

Almost surely, 2^21685687 is an apocalyptic number.

21685687 is a deficient number, since it is larger than the sum of its proper divisors (1).

21685687 is an equidigital number, since it uses as much as digits as its factorization.

21685687 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 161280, while the sum is 43.

The square root of 21685687 is about 4656.7893446021. The cubic root of 21685687 is about 278.8631069206.

The spelling of 21685687 in words is "twenty-one million, six hundred eighty-five thousand, six hundred eighty-seven".