In mathematics, infinitary combinatorics, or combinatorial set theory, is an extension of ideas in combinatorics to infinite sets. Some of the things studied include continuous graphs and trees, extensions of Ramsey's theorem, and Martin's axiom. Recent developments concern combinatorics of the continuum and combinatorics on successors of singular cardinals.
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Ramsey theory for infinite sets
Write κ, λ for ordinals, m for a cardinal number and n for a natural number. Erdős & Rado (1956) introduced the notation
as a shorthand way of saying that every partition of the set [κ]n of n-element subsets of
Assuming the axiom of choice, there are no ordinals κ with κ→(ω)ω, so n is usually taken to be finite. An extension where n is almost allowed to be infinite is the notation
which is a shorthand way of saying that every partition of the set of finite subsets of κ into m pieces has a subset of order type λ such that for any finite n, all subsets of size n are in the same element of the partition. When m is 2 it is often omitted.
Another variation is the notation
which is a shorthand way of saying that every coloring of the set [κ]n of n-element subsets of κ with 2 colors has a subset of order type λ such that all elements of [λ]n have the first color, or a subset of order type μ such that all elements of [μ]n have the second color.
Some properties of this include: (in what follows
In choiceless universes, partition properties with infinite exponents may hold, and some of them are obtained as consequences of the axiom of determinacy (AD). For example, Donald A. Martin proved that AD implies
Large cardinals
Several large cardinal properties can be defined using this notation. In particular: