Infinitary combinatorics

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. 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. 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 n of n-element subsets of κ {displaystyle kappa } into m pieces has a homogeneous set of order type λ. A homogeneous set is in this case a subset of κ such that every n-element subset is in the same element of the partition. When m is 2 it is often omitted. 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 isthe 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.