IP set

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

The finite sums over 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 set. They require that FS(D) equals A instead of just being a subset.

The name IP-set was coined by Furstenberg and Weiss to abbreviate "Infinite-dimensional Parallelepiped".

Contents 1 Hindman's Theorem 2 Semigroups 3 See also 4 References

[edit] Hindman's Theorem

If is an IP set and , then at least one of the 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 partions 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.

[edit] 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.

[edit] See also

• Syndetic set
• Piecewise syndetic set
• Thick set

[edit] References

• V. Bergelson, I. J. H. Knutson, R. McCutcheon "Simultaneous diophantine approximation and VIP Systems" Acta Arith. 116, Academia Scientiarum Polona, (2005), 13-23
• V. Bergelson, "Minimal Idempotents and Ergodic Ramsey Theory" Topics in Dynamics and Ergodic Theory 8-39, London Math. Soc. Lecture Note Series 310, Cambridge Univ. Press, Cambridge, (2003)
• V. Bergelson, N. Hindman, "Partition regular structures contained in large sets are abundant" J. Comb. Theory (Series A) 93 (2001), pp. 18-36
• H. Furstenberg, B. Weiss, "Topological Dynamics and Combinatorial Number Theory", J. d'Analyse Math. 34 (1978), pp. 61-85
• J. McLeod, "Some Notions of Size in Partial Semigroups", Topology Proceedings, Vol. 25 (2000), pp. 317-332