21081685391 has 32 divisors (see below), whose sum is σ = 24981089280. Its totient is φ = 17603308800.

The previous prime is 21081685381. The next prime is 21081685399. The reversal of 21081685391 is 19358618012.

It is a cyclic number.

It is not a de Polignac number, because 21081685391 - 2¹⁸ = 21081423247 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21081685399) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 3581771 + ... + 3587651.

It is an arithmetic number, because the mean of its divisors is an integer number (780659040).

Almost surely, 2^21081685391 is an apocalyptic number.

21081685391 is a deficient number, since it is larger than the sum of its proper divisors (3899403889).

21081685391 is a wasteful number, since it uses less digits than its factorization.

21081685391 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 6567.

The product of its (nonzero) digits is 103680, while the sum is 44.

The spelling of 21081685391 in words is "twenty-one billion, eighty-one million, six hundred eighty-five thousand, three hundred ninety-one".