81638634991 has 2 divisors, whose sum is σ = 81638634992. Its totient is φ = 81638634990.

The previous prime is 81638634971. The next prime is 81638635081. The reversal of 81638634991 is 19943683618.

It is a weak prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-81638634991 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (81638634901) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40819317495 + 40819317496.

It is an arithmetic number, because the mean of its divisors is an integer number (40819317496).

Almost surely, 2^81638634991 is an apocalyptic number.

81638634991 is a deficient number, since it is larger than the sum of its proper divisors (1).

81638634991 is an equidigital number, since it uses as much as digits as its factorization.

81638634991 is an evil number, because the sum of its binary digits is even.

The product of its digits is 6718464, while the sum is 58.

The spelling of 81638634991 in words is "eighty-one billion, six hundred thirty-eight million, six hundred thirty-four thousand, nine hundred ninety-one".