# Multiset

In mathematics, a multiset (sometimes also called a bag) differs from a set in that each member has a multiplicity, which is a natural number indicating (loosely speaking) how many times it is a member, or perhaps how many memberships it has in the multiset. For example, in the multiset { a, a, b, b, b, c }, the multiplicities of the members a, b, and c are respectively 2, 3, and 1.

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## Formal definition

Within set theory, a multiset can be formally defined as a pair (A, m) where A is some set and m : AN is a function from A to the set N of (positive) natural numbers. The set A is called the underlying set of elements. For each a in A the multiplicity of a is the number m(a).

It is common to write the function m as a set of ordered pairs {(a, m(a)) : aA} — indeed, this is the set-theoretic definition of the function m. For example,

• the multiset written as {a, b, b} is defined as {(a, 1), (b, 2)},
• likewise {a, a, b} is defined as {(a, 2), (b, 1)}, and
• the multiset {a, b} is defined as {(a, 1), (b, 1)}.

If the set A is finite, the size or length of the multiset (A, m) is the sum of all multiplicities for each element of A:

[itex]\sum_{a\in A}m(a).[itex]

One often says that this is the size of A counted with multiplicity.

A submultiset (B, n) of a multiset (A, m) is a subset BA and a function n : BN such that n(a) ≤ m(a).

## Examples

One of the most natural and simple examples is the multiset of prime factors of a number n. Here the underlying set of elements is the set of prime divisors of n. For example the 120 has the prime factorization

[itex]120 = 2^3 3^1 5^1[itex]

which gives the multiset {2, 2, 2, 3, 5}.

Another is the multiset of solutions of an algebraic equation. Everyone learns in secondary school that a quadratic equation has two solutions, but in some cases they are both the same number. Thus the multiset of solutions of the equation could be { 3, 5 }, or it could be { 4, 4 }. In the latter case it has a solution of multiplicity 2.

## Operations

The usual set operations such as union, intersection and Cartesian product can be easily generalized for multisets.

Suppose (A, m) and (B, n) are multisets

• The union can be defined as (AB, f) where f(x) = m(x) + n(x).
• The intersection can be defined as (AB, f) where f(x) = min{m(x), n(x)}.
• The cartesian product can be defined as (A × B, f) where f((x,y)) = m(x)n(y).

## Multiset coefficients

The number of submultisets of size k in a set of size n is the multiset coefficient

[itex]\left\langle \begin{matrix}n \\ k \end{matrix}\right\rangle = {n + k - 1 \choose n-1}={n+k-1 \choose k},[itex]

where the expressions to the right of "=" are binomial coefficients, i.e., the number of such multisets is the same as the number of subsets of size k in a set of size n + k − 1. Unlike the situation with sets, this cardinality will not be 0 when k > n. One simple way to prove this involves representing multisets in the following way. First, consider the notation for multisets that would represent { a, a, a, a, a, a, b, b, c, c, c, d, d, d, d, d, d, d } (6 as, 2 bs, 3 cs, 7 ds) in this form:

[itex]\bullet \bullet \bullet \bullet \bullet \bullet \mid \bullet \bullet \mid \bullet \bullet \bullet \mid \bullet \bullet \bullet \bullet \bullet \bullet \bullet [itex]

This is a multiset of size 18 made of elements of a set of size 4. The number of characters including both dots and vertical lines used in this notation is 18 + 4 − 1. The number of vertical lines is 4 − 1. The number of multisets of size 18 is then the number of ways to arrange the 4 − 1 vertical lines among the 18 + 4 − 1 characters, and is thus the number of subsets of size 4 − 1 in a set of size 18 + 4 − 1. Equivalently, it is the number of ways to arrange the 18 dots among the 18 + 4 − 1 characters, which is the number of subsets of size 18 of a set of size 18 + 4 − 1. This is

[itex]{18+4-1 \choose 4-1}={18+4-1 \choose 18},[itex]

so that is the value of the multiset coefficient

[itex]\left\langle\begin{matrix} 4 \\ 18 \end{matrix}\right\rangle.[itex]

One may define a generalized binomial coefficient

[itex]{n \choose k}={n(n-1)(n-2)\cdots(n-k+1) \over k!}[itex]

in which n is not required to be a nonnegative integer, but may be negative or a non-integer, or a non-real complex number. (If k = 0, then the value of this coefficient is 1 because it is the product of no numbers.) Then the number of multisets of size k in a set of size n is

[itex]\left\langle\begin{matrix} n \\ k \end{matrix}\right\rangle=(-1)^k{-n \choose k}.[itex]

This fact led Gian-Carlo Rota to ask "Why are negative sets multisets?". He considered that question worthy of the attention of philosophers of mathematics.

## Free commutative monoids

There is a connection with the free object concept: the free commutative monoid on a set X can be taken to be the set of finite multisets with elements drawn from X, with the obvious addition operation.de:Multimenge nl:Multiset pl:Multizbiór sl:večkratna množica

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