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Combinatorial properties of infinite sets. This is a corner-point of set theory and combinatorics.

4 votes
1 answer
367 views

When is it true that if $G$ is isomorphic to a spanning subgraph of $H$ and vice versa, then...

When is it true that if $G$ is isomorphic to a spanning subgraph of $H$ and $H$ is isomorphic to a spanning subgraph of $G$, then $G$ is isomorphic to $H$? Clearly this is true if $G$ and $H$ are fini …
Louis D's user avatar
  • 1,701
2 votes
Accepted

Injective choice function for infinite complete linear hypergraphs

After the discussion above, here is what I think is the cleanest proof and it has the property that $f$ is bijection (unless there is an edge of order 1). If there is an edge of order 1, then we must …
Louis D's user avatar
  • 1,701
1 vote

Graph $G$ such that removing an edge leaves $G$ "unchanged"

How about a graph $G=(V,E)$ consisting of infinitely many isolated vertices and infinitely many disjoint edges. Like the random graph it has the property that for all $e\in E$, $G\simeq (V, E\setminu …
Louis D's user avatar
  • 1,701
3 votes

Induced subgraphs of the almost-disjointness graph

My first thought for the case where $|V|\leq \aleph_0$ is that surely the Rado graph can be constructed as an induced subgraph of $([\omega]^{\omega}, E)$ (since the Rado graph contains a copy of ever …
Louis D's user avatar
  • 1,701
3 votes
1 answer
292 views

Perfect matchings in infinite regular bipartite graphs

This question was motivated by a discussion here and is related to a previous question here. Let $\kappa$ and $\lambda$ be cardinals such that $0<\lambda\leq \kappa$. Let $G=(A\cup B, E)$ be a bipart …
Louis D's user avatar
  • 1,701
4 votes
Accepted

Is König's Property for graphs inheritable from finite subgraphs?

(Just making my comment an answer as suggested.) If every finite subgraph of $G$ satisfies Kőnig's Property, then $G$ has no odd cycles and is thus bipartite. Aharoni (König's Duality Theorem For Infi …
Louis D's user avatar
  • 1,701