81601991 has 2 divisors, whose sum is σ = 81601992. Its totient is φ = 81601990.

The previous prime is 81601987. The next prime is 81602029. The reversal of 81601991 is 19910618.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 81601991 - 2² = 81601987 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (81601901) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40800995 + 40800996.

It is an arithmetic number, because the mean of its divisors is an integer number (40800996).

Almost surely, 2^81601991 is an apocalyptic number.

81601991 is a deficient number, since it is larger than the sum of its proper divisors (1).

81601991 is an equidigital number, since it uses as much as digits as its factorization.

81601991 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 3888, while the sum is 35.

The square root of 81601991 is about 9033.3820355391. The cubic root of 81601991 is about 433.7441047072.

The spelling of 81601991 in words is "eighty-one million, six hundred one thousand, nine hundred ninety-one".