81508561261 has 4 divisors (see below), whose sum is σ = 81603671292. Its totient is φ = 81413451232.

The previous prime is 81508561243. The next prime is 81508561271. The reversal of 81508561261 is 16216580518.

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 57852275625 + 23656285636 = 240525^2 + 153806^2 .

It is a cyclic number.

It is not a de Polignac number, because 81508561261 - 2³⁵ = 47148822893 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81508561241) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 47553730 + ... + 47555443.

It is an arithmetic number, because the mean of its divisors is an integer number (20400917823).

Almost surely, 2^81508561261 is an apocalyptic number.

It is an amenable number.

81508561261 is a deficient number, since it is larger than the sum of its proper divisors (95110031).

81508561261 is an equidigital number, since it uses as much as digits as its factorization.

81508561261 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 95110030.

The product of its (nonzero) digits is 115200, while the sum is 43.

The spelling of 81508561261 in words is "eighty-one billion, five hundred eight million, five hundred sixty-one thousand, two hundred sixty-one".