81205977691 has 2 divisors, whose sum is σ = 81205977692. Its totient is φ = 81205977690.

The previous prime is 81205977649. The next prime is 81205977701. The reversal of 81205977691 is 19677950218.

It is a happy number.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 81205977691 - 2²⁷ = 81071759963 is a prime.

It is not a weakly prime, because it can be changed into another prime (81205977631) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40602988845 + 40602988846.

It is an arithmetic number, because the mean of its divisors is an integer number (40602988846).

Almost surely, 2^81205977691 is an apocalyptic number.

81205977691 is a deficient number, since it is larger than the sum of its proper divisors (1).

81205977691 is an equidigital number, since it uses as much as digits as its factorization.

81205977691 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 1905120, while the sum is 55.

The spelling of 81205977691 in words is "eighty-one billion, two hundred five million, nine hundred seventy-seven thousand, six hundred ninety-one".