11781 has 24 divisors (see below), whose sum is σ = 22464. Its totient is φ = 5760.

The previous prime is 11779. The next prime is 11783. The reversal of 11781 is 18711.

11781 = T₁₅ + T₁₆ + ... + T₄₁.

11781 is a nontrivial binomial coefficient, being equal to C(154, 2).

It is an interprime number because it is at equal distance from previous prime (11779) and next prime (11783).

It is not a de Polignac number, because 11781 - 2¹ = 11779 is a prime.

It is a super-2 number, since 2×11781² = 277583922, which contains 22 as substring.

It is an Ulam number.

It is a Curzon number.

11781 is a lucky number.

It is a plaindrome in base 12.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (11783) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 685 + ... + 701.

It is an arithmetic number, because the mean of its divisors is an integer number (936).

2¹¹⁷⁸¹ is an apocalyptic number.

11781 is a gapful number since it is divisible by the number (11) formed by its first and last digit.

11781 is the 153-rd triangular number, the 77-th hexagonal number and also the 63-rd octagonal number.

It is an amenable number.

11781 is a deficient number, since it is larger than the sum of its proper divisors (10683).

11781 is a wasteful number, since it uses less digits than its factorization.

11781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 41 (or 38 counting only the distinct ones).

The product of its digits is 56, while the sum is 18.

The square root of 11781 is about 108.5403150908. The cubic root of 11781 is about 22.7541553413.

The spelling of 11781 in words is "eleven thousand, seven hundred eighty-one".