# Questions tagged [infinitary-combinatorics]

For topics of a combinatorial character in set theory. Topics belonging to "combinatorial set theory" may be tagged this way. These include: Partition calculus, diamond principles, square principles, combinatorial properties of infinite graphs or partial orders, etc.

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### What's wrong with this proof that $\mathbb{R}$ cannot be well-ordered?

I'm trying to figure out where I'm going wrong with the following proof that $\mathbb{R}$ cannot be well ordered (and thus, ZFC is inconsistent). It's based off a special case of the Erdős–Dushnik–...
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### Uncountable Intersection of uncountably many uncountable subsets of $\mathbb{R}$

I'm trying to prove or find a counterexample to the following: If $A$ is an uncountable collection of uncountable subsets $S$ of $\mathbb{R}$ such that each $x\in\mathbb{R}$ is in uncountably many of ...
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### Schur function in linerly shifted power symmetric basis

Previously I have aksed the following question about schur function in power symmetric basis Schur function principal specialisation Let $s_{\lambda}(p_1,p_2,)$ denote schur function in power-sum ...
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### On the distributive number $\mathfrak h$

The distributive number $\mathfrak h$ is defined as the least cardinal $\kappa$ such that there exists a family of $\kappa$ open dense subsets in the preordered set $([\omega]^\omega,\subset^*)$ whose ...
2answers
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### How is $\sum_{i=0}^{\infty}\sum_{j=0}^{\infty}x^{i+j} = \sum_{n=0}^{\infty}\sum_{i=0}^{n}x^n$

My professor used this in class for a proof and I'm having trouble understanding it. $\sum_{i=0}^{\infty}\sum_{j=0}^{\infty}x^{i+j} = \sum_{n=0}^{\infty}\sum_{i=0}^{n}x^n$ The way he explained it, ...
1answer
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### Partitioning an infinite set into fixed number of sets

Suppose we have a set of size $\kappa$, and want to partition it into $\mu$ sets, where $\kappa$ is an infinite cardinal, and $1<\mu\leq\kappa$. I am aware that it can be done in $2^\kappa$ ways (...
1answer
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### the generalized Delta System Lemma

I'm trying to understand the proof of the generalized Delta System Lemma in Kunen's Set Theory, the 2013 edition. This stationary set $T$ we've constructed using Fodor's Lemma (Pressing Down Lemma) ...
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### Infinite rooms and doors (2)

Suppose we have a house (with finitely many rooms) in which every room has an even number of doors. Prove that the number of doors from the house to the outside world is also even. All I could figure ...
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### Existence of Aronszajn tree

There are theories that prove the existence and non-existence of Souslin trees [exist if $V=L$, don't exist if $\mathsf{MA}(\aleph_1)$] and Kurepa trees [exist if $V=L$, don't exist by Lévy collapsing ...
1answer
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### How to understand $e$ and other terms in this probability distribution?

Algebraically, the derivation of the probability function of a Poisson random variable is fairly straightforward. Letting $\mu = np$ with $n \to \infty, p \to 0$ so that $\mu < \infty$: Starting ...
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### number of finite subsets of a cardinal $\kappa$

Why there are only $\kappa$ many finite subsets of a cardinal $\kappa$? There are obviously at least $\kappa$ of them, but why also at most $\kappa$ of them?
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### Reference Request: A Tychonoff space $X$ where the game $\mathrm{G}_1(\Omega,\Omega)$ is undetermined

Given a Tychonoff space $X$, let $\Omega$ be the set of all $\omega$-coverings of $X$, where by $\omega$-cover we mean a collection $\mathcal{U}$ of open sets of $X$ such that for any \$F\in[X]^{<\...