81781 has 6 divisors (see below), whose sum is σ = 95190. Its totient is φ = 70056.

The previous prime is 81773. The next prime is 81799. The reversal of 81781 is 18718.

81781 is nontrivially palindromic in base 6.

It can be written as a sum of positive squares in only one way, i.e., 70756 + 11025 = 266^2 + 105^2 .

It is not a de Polignac number, because 81781 - 2³ = 81773 is a prime.

It is a super-2 number, since 2×81781² = 13376263922, which contains 22 as substring.

It is a Duffinian number.

81781 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81701) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 786 + ... + 883.

It is an arithmetic number, because the mean of its divisors is an integer number (15865).

2⁸¹⁷⁸¹ is an apocalyptic number.

It is an amenable number.

81781 is a deficient number, since it is larger than the sum of its proper divisors (13409).

81781 is a wasteful number, since it uses less digits than its factorization.

81781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1683 (or 1676 counting only the distinct ones).

The product of its digits is 448, while the sum is 25.

The square root of 81781 is about 285.9737750214. The cubic root of 81781 is about 43.4061038935.

It can be divided in two parts, 817 and 81, that added together give a palindrome (898).

The spelling of 81781 in words is "eighty-one thousand, seven hundred eighty-one".