28781 has 4 divisors (see below), whose sum is σ = 30492. Its totient is φ = 27072.

The previous prime is 28771. The next prime is 28789. The reversal of 28781 is 18782.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 18782 = 2 ⋅9391.

It can be written as a sum of positive squares in 2 ways, for example, as 16900 + 11881 = 130^2 + 109^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-28781 is a prime.

It is a super-2 number, since 2×28781² = 1656691922, which contains 22 as substring.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 28781.

It is a congruent number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 28781.

It is not an unprimeable number, because it can be changed into a prime (28789) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 830 + ... + 863.

It is an arithmetic number, because the mean of its divisors is an integer number (7623).

2²⁸⁷⁸¹ is an apocalyptic number.

It is an amenable number.

28781 is a deficient number, since it is larger than the sum of its proper divisors (1711).

28781 is a wasteful number, since it uses less digits than its factorization.

28781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1710.

The product of its digits is 896, while the sum is 26.

The square root of 28781 is about 169.6496389622. The cubic root of 28781 is about 30.6456351281.

Subtracting from 28781 its reverse (18782), we obtain a palindrome (9999).

The spelling of 28781 in words is "twenty-eight thousand, seven hundred eighty-one".