16581 has 4 divisors (see below), whose sum is σ = 22112. Its totient is φ = 11052.

The previous prime is 16573. The next prime is 16603. The reversal of 16581 is 18561.

16581 is digitally balanced in base 3, because in such base it contains all the possibile digits an equal number of times.

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 not a de Polignac number, because 16581 - 2³ = 16573 is a prime.

It is a super-2 number, since 2×16581² = 549859122, which contains 22 as substring.

It is a D-number.

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

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (16561) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (5) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2761 + ... + 2766.

It is an arithmetic number, because the mean of its divisors is an integer number (5528).

2¹⁶⁵⁸¹ is an apocalyptic number.

It is an amenable number.

16581 is a deficient number, since it is larger than the sum of its proper divisors (5531).

16581 is an equidigital number, since it uses as much as digits as its factorization.

16581 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 5530.

The product of its digits is 240, while the sum is 21.

The square root of 16581 is about 128.7672318566. The cubic root of 16581 is about 25.4998077648.

The spelling of 16581 in words is "sixteen thousand, five hundred eighty-one".