81856209991 has 2 divisors, whose sum is σ = 81856209992. Its totient is φ = 81856209990.

The previous prime is 81856209989. The next prime is 81856210021. The reversal of 81856209991 is 19990265818.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 81856209991 - 2¹ = 81856209989 is a prime.

It is a super-2 number, since 2×81856209991² (a number of 23 digits) contains 22 as substring.

Together with 81856209989, it forms a pair of twin primes.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (81856209901) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40928104995 + 40928104996.

It is an arithmetic number, because the mean of its divisors is an integer number (40928104996).

Almost surely, 2^81856209991 is an apocalyptic number.

81856209991 is a deficient number, since it is larger than the sum of its proper divisors (1).

81856209991 is an equidigital number, since it uses as much as digits as its factorization.

81856209991 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 2799360, while the sum is 58.

The spelling of 81856209991 in words is "eighty-one billion, eight hundred fifty-six million, two hundred nine thousand, nine hundred ninety-one".