21686591 has 2 divisors, whose sum is σ = 21686592. Its totient is φ = 21686590.

The previous prime is 21686579. The next prime is 21686617. The reversal of 21686591 is 19568612.

It is a weak prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21686591 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21686501) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10843295 + 10843296.

It is an arithmetic number, because the mean of its divisors is an integer number (10843296).

Almost surely, 2^21686591 is an apocalyptic number.

21686591 is a deficient number, since it is larger than the sum of its proper divisors (1).

21686591 is an equidigital number, since it uses as much as digits as its factorization.

21686591 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 25920, while the sum is 38.

The square root of 21686591 is about 4656.8864061731. The cubic root of 21686591 is about 278.8669818074.

The spelling of 21686591 in words is "twenty-one million, six hundred eighty-six thousand, five hundred ninety-one".