81991157 has 2 divisors, whose sum is σ = 81991158. Its totient is φ = 81991156.

The previous prime is 81991141. The next prime is 81991163. The reversal of 81991157 is 75119918.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 71808676 + 10182481 = 8474^2 + 3191^2 .

It is a cyclic number.

It is not a de Polignac number, because 81991157 - 2⁴ = 81991141 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (81991127) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40995578 + 40995579.

It is an arithmetic number, because the mean of its divisors is an integer number (40995579).

Almost surely, 2^81991157 is an apocalyptic number.

It is an amenable number.

81991157 is a deficient number, since it is larger than the sum of its proper divisors (1).

81991157 is an equidigital number, since it uses as much as digits as its factorization.

81991157 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 22680, while the sum is 41.

The square root of 81991157 is about 9054.8968519801. The cubic root of 81991157 is about 434.4325308407.

The spelling of 81991157 in words is "eighty-one million, nine hundred ninety-one thousand, one hundred fifty-seven".