18781 has 4 divisors (see below), whose sum is σ = 21472. Its totient is φ = 16092.

The previous prime is 18773. The next prime is 18787.

18781 is nontrivially palindromic in base 10.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 18781 - 2³ = 18773 is a prime.

It is a super-2 number, since 2×18781² = 705451922, which contains 22 as substring.

It is an alternating number because its digits alternate between odd and even.

It is a Duffinian number.

It is a nialpdrome in base 12.

It is a zygodrome in base 5.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (18787) 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, 1335 + ... + 1348.

It is an arithmetic number, because the mean of its divisors is an integer number (5368).

2¹⁸⁷⁸¹ is an apocalyptic number.

It is an amenable number.

18781 is a deficient number, since it is larger than the sum of its proper divisors (2691).

18781 is an equidigital number, since it uses as much as digits as its factorization.

18781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2690.

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

The square root of 18781 is about 137.0437886225. The cubic root of 18781 is about 26.5810972417.

The spelling of 18781 in words is "eighteen thousand, seven hundred eighty-one".