IP set explained

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). Slightly more generally, for a sequence of natural numbers (n_i), one can consider the set of finite sums FS((n_i)), consisting of the sums of all finite length subsequences of (n_i).

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. Equivalently, one may require that A contains all finite sums FS((n_i)) of a sequence (n_i).

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

The term IP set was coined by Hillel Furstenberg and Benjamin Weiss^{[1] [2]} to abbreviate "infinite-dimensional parallelepiped". Serendipitously, the abbreviation IP can also be expanded to "idempotent"^[3] (a set is an 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.^{[4] [5]} In different terms, Hindman's theorem states that the class of IP sets is partition regular.

Since the set of natural numbers itself is an IP set and partitions can also be seen as colorings, one 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 color. 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 is named for mathematician Neil Hindman, who proved it in 1974.^[4] The Milliken–Taylor theorem is a common generalisation of Hindman's theorem and Ramsey's theorem.

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. A variant of Hindman's theorem is true for arbitrary semigroups.^{[6] [7]}

See also

• Ergodic Ramsey theory
• Piecewise syndetic set
• Syndetic set
• Thick set

Further reading

• Vitaly Bergelson, I. J. H. Knutson, R. McCutcheon "Simultaneous diophantine approximation and VIP Systems " Acta Arith. 116, Academia Scientiarum Polona, (2005), 13-23
• Vitaly 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)
• Bergelson . Vitaly . Vitaly Bergelson. Hindman . Neil . Neil Hindman. Partition regular structures contained in large sets are abundant. . Series A. 93. 1. 2001. 18–36. 10.1006/jcta.2000.3061 . free. September 18, 2022.
• J. McLeod, "Some Notions of Size in Partial Semigroups", Topology Proceedings, Vol. 25 (2000), pp. 317–332

Notes and References

1. Furstenberg . H. . Hillel Furstenberg . Weiss . B. . Benjamin Weiss . Topological Dynamics and Combinatorial Number Theory . . 34 . 1978 . 61–85 . 10.1007/BF02790008 . free.
2. Book: Harry, Furstenberg. Recurrence in ergodic theory and combinatorial number theory. July 2014. 9780691615363. Princeton, New Jersey. 889248822.
3. Bergelson. V.. Leibman. A.. 2016. Sets of large values of correlation functions for polynomial cubic configurations. Ergodic Theory and Dynamical Systems. 38. 2. 499–522. 10.1017/etds.2016.49. 31083478. 0143-3857.
4. Hindman . Neil . Neil Hindman . 1974 . Finite sums from sequences within cells of a partition of N . . Series A . 17 . 1 . 1–11 . 10.1016/0097-3165(74)90023-5 . free . free . 10338.dmlcz/127803.
5. Baumgartner. James E. A short proof of Hindman's theorem. . Series A. 17. 3. 384–386. 10.1016/0097-3165(74)90103-4. 1974. free.
6. Golan. Gili. Tsaban. Boaz. Hindmanʼs coloring theorem in arbitrary semigroups. Journal of Algebra. 395. 111–120. 10.1016/j.jalgebra.2013.08.007. free. 1303.3600. 2013. 11437903.
7. Book: Algebra in the Stone-Čech compactification : theory and applications. Hindman. Neil . Neil Hindman. Strauss. Dona. Dona Strauss . 1998. Walter de Gruyter. 311015420X. New York. 39368501.