81991891 has 4 divisors (see below), whose sum is σ = 82028232. Its totient is φ = 81955552.

The previous prime is 81991883. The next prime is 81991913. The reversal of 81991891 is 19819918.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 81991891 - 2³ = 81991883 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81991831) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 14545 + ... + 19378.

It is an arithmetic number, because the mean of its divisors is an integer number (20507058).

Almost surely, 2^81991891 is an apocalyptic number.

81991891 is a deficient number, since it is larger than the sum of its proper divisors (36341).

81991891 is a wasteful number, since it uses less digits than its factorization.

81991891 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 36340.

The product of its digits is 46656, while the sum is 46.

The square root of 81991891 is about 9054.9373824450. The cubic root of 81991891 is about 434.4338272103.

The spelling of 81991891 in words is "eighty-one million, nine hundred ninety-one thousand, eight hundred ninety-one".