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

8 votes
1 answer
228 views

Aronszajn Trees when AC fails

This question may be easy and indicative of my ignorance about the failure of the axiom of choice. If so, I apologize. Below assume $\mathsf{DC}$ but not $\mathsf{AC}$. Suppose we have a partial order …
Corey Bacal Switzer's user avatar
11 votes
1 answer
478 views

Ways to add Aronszajn trees which are neither Souslin nor special

By an Aronszajn tree, I mean a tree of height $\omega_1$ with countable levels and no branch. Such a tree is Souslin if it has no uncountable antichains and special if it can be written as the countab …
Corey Bacal Switzer's user avatar
3 votes
1 answer
206 views

Does the Hawaiian Earring Group embed into the permutation group of $\mathbb N$?

Recall that the Hawaiian earring group, $\mathbb G$, is the fundamental group of the Hawaiian Earing using the point at the origin. It can be understood more combinatorially as a subgroup of the inver …
Corey Bacal Switzer's user avatar