81856209989 has 2 divisors, whose sum is σ = 81856209990. Its totient is φ = 81856209988.

The previous prime is 81856209979. The next prime is 81856209991. The reversal of 81856209989 is 98990265818.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 81390384100 + 465825889 = 285290^2 + 21583^2 .

It is a cyclic number.

It is not a de Polignac number, because 81856209989 - 2²⁸ = 81587774533 is a prime.

It is a super-2 number, since 2×81856209989² (a number of 23 digits) contains 22 as substring.

Together with 81856209991, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (81856209979) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40928104994 + 40928104995.

It is an arithmetic number, because the mean of its divisors is an integer number (40928104995).

Almost surely, 2^81856209989 is an apocalyptic number.

It is an amenable number.

81856209989 is a deficient number, since it is larger than the sum of its proper divisors (1).

81856209989 is an equidigital number, since it uses as much as digits as its factorization.

81856209989 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 22394880, while the sum is 65.

The spelling of 81856209989 in words is "eighty-one billion, eight hundred fifty-six million, two hundred nine thousand, nine hundred eighty-nine".