Partition regularity

In combinatorics, a branch of mathematics, partition regularity is one notion of largeness for a collection of sets.

Given a set , a collection of subsets is called partition regular if every set A in the collection has the property that, no matter how A is partitioned into finitely many subsets, at least one of the subsets will also belong to the collection. That is, for any , and any finite partition , there exists an i ≤ n, such that belongs to . Ramsey theory is sometimes characterized as the study of which collections are partition regular.

ExamplesEdit

  • the collection of all infinite subsets of an infinite set X is a prototypical example. In this case partition regularity asserts that every finite partition of an infinite set has an infinite cell (i.e. the infinite pigeonhole principle.)
  • sets with positive upper density in  : the upper density   of   is defined as   (Szemerédi's theorem)
  • For any ultrafilter   on a set  ,   is partition regular: for any  , if  , then exactly one  .
  • sets of recurrence: a set R of integers is called a set of recurrence if for any measure preserving transformation   of the probability space (Ω, β, μ) and   of positive measure there is a nonzero   so that  .
  • Call a subset of natural numbers a.p.-rich if it contains arbitrarily long arithmetic progressions. Then the collection of a.p.-rich subsets is partition regular (Van der Waerden, 1927).
  • Let   be the set of all n-subsets of  . Let  . For each n,   is partition regular. (Ramsey, 1930).
  • For each infinite cardinal  , the collection of stationary sets of   is partition regular. More is true: if   is stationary and   for some  , then some   is stationary.
  • the collection of  -sets:   is a  -set if   contains the set of differences   for some sequence  .
  • the set of barriers on  : call a collection   of finite subsets of   a barrier if:
    •   and
    • for all infinite  , there is some   such that the elements of X are the smallest elements of I; i.e.   and  .
This generalizes Ramsey's theorem, as each   is a barrier. (Nash-Williams, 1965)
  • finite products of infinite trees (Halpern–Läuchli, 1966)
  • piecewise syndetic sets (Brown, 1968)
  • Call a subset of natural numbers i.p.-rich if it contains arbitrarily large finite sets together with all their finite sums. Then the collection of i.p.-rich subsets is partition regular (FolkmanRado–Sanders, 1968).
  • (m, p, c)-sets (Deuber, 1973)
  • IP sets (Hindman, 1974, see also Hindman, Strauss, 1998)
  • MTk sets for each k, i.e. k-tuples of finite sums (Milliken–Taylor, 1975)
  • central sets; i.e. the members of any minimal idempotent in  , the Stone–Čech compactification of the integers. (Furstenberg, 1981, see also Hindman, Strauss, 1998)

ReferencesEdit

  1. Vitaly Bergelson, N. Hindman Partition regular structures contained in large sets are abundant J. Comb. Theory A 93 (2001), 18–36.
  2. T. Brown, An interesting combinatorial method in the theory of locally finite semigroups, Pacific J. Math. 36, no. 2 (1971), 285–289.
  3. W. Deuber, Mathematische Zeitschrift 133, (1973) 109–123
  4. N. Hindman, Finite sums from sequences within cells of a partition of N, J. Comb. Theory A 17 (1974) 1–11.
  5. C.St.J.A. Nash-Williams, On well-quasi-ordering transfinite sequences, Proc. Camb. Phil. Soc. 61 (1965), 33–39.
  6. N. Hindman, D. Strauss, Algebra in the Stone–Čech compactification, De Gruyter, 1998
  7. J.Sanders, A Generalization of Schur's Theorem, Doctoral Dissertation, Yale University, 1968.