81815891 has 2 divisors, whose sum is σ = 81815892. Its totient is φ = 81815890.

The previous prime is 81815879. The next prime is 81815893. The reversal of 81815891 is 19851818.

It is a happy number.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 81815891 - 2¹⁰ = 81814867 is a prime.

Together with 81815893, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (81815893) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40907945 + 40907946.

It is an arithmetic number, because the mean of its divisors is an integer number (40907946).

Almost surely, 2^81815891 is an apocalyptic number.

81815891 is a deficient number, since it is larger than the sum of its proper divisors (1).

81815891 is an equidigital number, since it uses as much as digits as its factorization.

81815891 is an evil number, because the sum of its binary digits is even.

The product of its digits is 23040, while the sum is 41.

The square root of 81815891 is about 9045.2137067070. The cubic root of 81815891 is about 434.1227593444.

The spelling of 81815891 in words is "eighty-one million, eight hundred fifteen thousand, eight hundred ninety-one".