81555643661 has 2 divisors, whose sum is σ = 81555643662. Its totient is φ = 81555643660.

The previous prime is 81555643649. The next prime is 81555643691. The reversal of 81555643661 is 16634655518.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 60179449225 + 21376194436 = 245315^2 + 146206^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-81555643661 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (81555643621) 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, 40777821830 + 40777821831.

It is an arithmetic number, because the mean of its divisors is an integer number (40777821831).

Almost surely, 2^81555643661 is an apocalyptic number.

It is an amenable number.

81555643661 is a deficient number, since it is larger than the sum of its proper divisors (1).

81555643661 is an equidigital number, since it uses as much as digits as its factorization.

81555643661 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 2592000, while the sum is 50.

The spelling of 81555643661 in words is "eighty-one billion, five hundred fifty-five million, six hundred forty-three thousand, six hundred sixty-one".