81556445921 has 2 divisors, whose sum is σ = 81556445922. Its totient is φ = 81556445920.

The previous prime is 81556445917. The next prime is 81556445923. The reversal of 81556445921 is 12954465518.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 80592964321 + 963481600 = 283889^2 + 31040^2 .

It is a cyclic number.

It is not a de Polignac number, because 81556445921 - 2² = 81556445917 is a prime.

Together with 81556445923, it forms a pair of twin primes.

It is a Chen prime.

It is a self number, because there is not a number n which added to its sum of digits gives 81556445921.

It is not a weakly prime, because it can be changed into another prime (81556445923) 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 as a sum of consecutive naturals, namely, 40778222960 + 40778222961.

It is an arithmetic number, because the mean of its divisors is an integer number (40778222961).

Almost surely, 2^81556445921 is an apocalyptic number.

It is an amenable number.

81556445921 is a deficient number, since it is larger than the sum of its proper divisors (1).

81556445921 is an equidigital number, since it uses as much as digits as its factorization.

81556445921 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 1728000, while the sum is 50.

The spelling of 81556445921 in words is "eighty-one billion, five hundred fifty-six million, four hundred forty-five thousand, nine hundred twenty-one".