81991 has 16 divisors (see below), whose sum is σ = 108864. Its totient is φ = 59904.

The previous prime is 81973. The next prime is 82003. The reversal of 81991 is 19918.

It is not a de Polignac number, because 81991 - 2¹¹ = 79943 is a prime.

It is a plaindrome in base 11.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81901) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 1521 + ... + 1573.

It is an arithmetic number, because the mean of its divisors is an integer number (6804).

2⁸¹⁹⁹¹ is an apocalyptic number.

81991 is a deficient number, since it is larger than the sum of its proper divisors (26873).

81991 is a wasteful number, since it uses less digits than its factorization.

81991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 90.

The product of its digits is 648, while the sum is 28.

The square root of 81991 is about 286.3407061527. The cubic root of 81991 is about 43.4432253551.

It can be divided in two parts, 819 and 91, that multiplied together give a square (74529 = 273²).

The spelling of 81991 in words is "eighty-one thousand, nine hundred ninety-one".