81691 has 4 divisors (see below), whose sum is σ = 82384. Its totient is φ = 81000.

The previous prime is 81689. The next prime is 81701. The reversal of 81691 is 19618.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It is a 4-Lehmer number, since φ(81691) divides (81691-1)⁴.

It is a cyclic number.

It is not a de Polignac number, because 81691 - 2¹ = 81689 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81611) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 120 + ... + 421.

It is an arithmetic number, because the mean of its divisors is an integer number (20596).

2⁸¹⁶⁹¹ is an apocalyptic number.

81691 is a deficient number, since it is larger than the sum of its proper divisors (693).

81691 is a wasteful number, since it uses less digits than its factorization.

81691 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 692.

The product of its digits is 432, while the sum is 25.

The square root of 81691 is about 285.8163746184. The cubic root of 81691 is about 43.3901752414.

The spelling of 81691 in words is "eighty-one thousand, six hundred ninety-one".