11581 has 4 divisors (see below), whose sum is σ = 11932. Its totient is φ = 11232.

The previous prime is 11579. The next prime is 11587. The reversal of 11581 is 18511.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 18511 = 107 ⋅173.

It can be written as a sum of positive squares in 2 ways, for example, as 4356 + 7225 = 66^2 + 85^2 .

It is a cyclic number.

It is not a de Polignac number, because 11581 - 2¹ = 11579 is a prime.

It is a super-2 number, since 2×11581² = 268239122, which contains 22 as substring.

It is a Duffinian number.

11581 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (11587) 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, 120 + ... + 193.

It is an arithmetic number, because the mean of its divisors is an integer number (2983).

2¹¹⁵⁸¹ is an apocalyptic number.

It is an amenable number.

11581 is a deficient number, since it is larger than the sum of its proper divisors (351).

11581 is an equidigital number, since it uses as much as digits as its factorization.

11581 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 350.

The product of its digits is 40, while the sum is 16.

The square root of 11581 is about 107.6150547089. The cubic root of 11581 is about 22.6246578706.

It can be divided in two parts, 115 and 81, that added together give a square (196 = 14²).

The spelling of 11581 in words is "eleven thousand, five hundred eighty-one".