81891 has 12 divisors (see below), whose sum is σ = 123032. Its totient is φ = 54432.

The previous prime is 81883. The next prime is 81899. The reversal of 81891 is 19818.

It is an interprime number because it is at equal distance from previous prime (81883) and next prime (81899).

It is not a de Polignac number, because 81891 - 2³ = 81883 is a prime.

It is a super-2 number, since 2×81891² = 13412271762, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81899) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 75 + ... + 411.

2⁸¹⁸⁹¹ is an apocalyptic number.

81891 is a gapful number since it is divisible by the number (81) formed by its first and last digit.

81891 is a deficient number, since it is larger than the sum of its proper divisors (41141).

81891 is an equidigital number, since it uses as much as digits as its factorization.

81891 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 352 (or 340 counting only the distinct ones).

The product of its digits is 576, while the sum is 27.

The square root of 81891 is about 286.1660357205. The cubic root of 81891 is about 43.4255563836.

Adding to 81891 its sum of digits (27), we get a palindrome (81918).

It can be divided in two parts, 818 and 91, that added together give a palindrome (909).

The spelling of 81891 in words is "eighty-one thousand, eight hundred ninety-one".