27781 has 4 divisors (see below), whose sum is σ = 29932. Its totient is φ = 25632.

The previous prime is 27779. The next prime is 27791. The reversal of 27781 is 18772.

It is a happy number.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 2500 + 25281 = 50^2 + 159^2 .

It is a cyclic number.

It is not a de Polignac number, because 27781 - 2¹ = 27779 is a prime.

It is a super-2 number, since 2×27781² = 1543567922, which contains 22 as substring.

It is a Duffinian number.

It is a nialpdrome in base 13.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (27701) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1056 + ... + 1081.

It is an arithmetic number, because the mean of its divisors is an integer number (7483).

2²⁷⁷⁸¹ is an apocalyptic number.

It is an amenable number.

27781 is a deficient number, since it is larger than the sum of its proper divisors (2151).

27781 is a wasteful number, since it uses less digits than its factorization.

27781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2150.

The product of its digits is 784, while the sum is 25.

The square root of 27781 is about 166.6763330530. The cubic root of 27781 is about 30.2865142019.

Subtracting from 27781 its reverse (18772), we obtain a palindrome (9009).

It can be divided in two parts, 27 and 781, that added together give a palindrome (808).

The spelling of 27781 in words is "twenty-seven thousand, seven hundred eighty-one".