# IP set

In mathematics, an IP set is a set of natural numbers which contains all finite sums of some infinite set.

The finite sums of a set D of natural numbers are all those numbers that can be obtained by adding up the elements of some finite nonempty subset of D. The set of all finite sums over D is often denoted as FS(D).

A set A of natural numbers is an IP set if there exists an infinite set D such that FS(D) is a subset of A.

Some authors give a slightly different definition of IP sets. They require that FS(D) equal A instead of just being a subset.

Sources disagree on the origin of the name IP set. Some claim it was coined by Furstenberg and Weiss to abbreviate "Infinite-dimensional Parallelepiped", others that it abbreviates "idempotent" (since a set is IP if and only if it is a member of an idempotent ultrafilter).

## Hindman's Theorem

If $S\,$ is an IP set and $S = C_1 \cup C_2 \cup ... \cup C_n$, then at least one $C_i\,$ is an IP set. This is known as Hindman's Theorem, or the Finite Sums Theorem.

Since the set of natural numbers itself is an IP-set and partitions can also be seen as colorings, we can reformulate a special case of Hindman's Theorem in more familiar terms: Suppose the natural numbers are "colored" with n different colors; each natural number gets one and only one of the n colors. Then there exists a color c and an infinite set D of natural numbers, all colored with c, such that every finite sum over D also has color c.

Hindman's Theorem states that the class of IP sets is partition regular.

## Semigroups

The definition of being IP has been extended from subsets of the special semigroup of natural numbers with addition to subsets of semigroups and partial semigroups in general.