81187991 has 2 divisors, whose sum is σ = 81187992. Its totient is φ = 81187990.

The previous prime is 81187979. The next prime is 81188017. The reversal of 81187991 is 19978118.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 81187991 - 2²² = 76993687 is a prime.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (81187921) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40593995 + 40593996.

It is an arithmetic number, because the mean of its divisors is an integer number (40593996).

Almost surely, 2^81187991 is an apocalyptic number.

81187991 is a deficient number, since it is larger than the sum of its proper divisors (1).

81187991 is an equidigital number, since it uses as much as digits as its factorization.

81187991 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 36288, while the sum is 44.

The square root of 81187991 is about 9010.4378916898. The cubic root of 81187991 is about 433.0093407909.

The spelling of 81187991 in words is "eighty-one million, one hundred eighty-seven thousand, nine hundred ninety-one".