81556817785 has 4 divisors (see below), whose sum is σ = 97868181348. Its totient is φ = 65245454224.

The previous prime is 81556817737. The next prime is 81556817791. The reversal of 81556817785 is 58771865518.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 28338018921 + 53218798864 = 168339^2 + 230692^2 .

It is a cyclic number.

It is not a de Polignac number, because 81556817785 - 2²¹ = 81554720633 is a prime.

It is a Duffinian number.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 8155681774 + ... + 8155681783.

It is an arithmetic number, because the mean of its divisors is an integer number (24467045337).

Almost surely, 2^81556817785 is an apocalyptic number.

It is an amenable number.

81556817785 is a deficient number, since it is larger than the sum of its proper divisors (16311363563).

81556817785 is a wasteful number, since it uses less digits than its factorization.

81556817785 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 16311363562.

The product of its digits is 18816000, while the sum is 61.

The spelling of 81556817785 in words is "eighty-one billion, five hundred fifty-six million, eight hundred seventeen thousand, seven hundred eighty-five".