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Constructing crowded Hausdorff $P$-spaces in set theory without the axiom of choice
Eleftherios Tachtsis, Eliza Wajch
https://arxiv.org/abs/2510.11935 https://

Constructing crowded Hausdorff $P$-spaces in set theory without the axiom of choice
For an infinite set $X$, a closed under finite unions family $\mathcal{Z}$ with $[X]^{<ω}\subseteq\mathcal{Z}\subseteq\mathcal{P}(X)$, and any $\mathcal{A}\subseteq\mathcal{P}(X)$, the topology $τ_{\mathcal{A}}[\mathcal{Z}]=\{V\in\mathcal{A}: (\forall x\in V)(\exists z\in \mathcal{Z})(x\cap z=\emptyset \wedge \{y\in\mathcal{A}: x\subseteq y\subseteq X\setminus z\}\subseteq V)\}$ on $\mathcal{A}$ is investigated to give answers to the following open problem in various models of $\mathbf{ZF}$ o…

Formal Cantor's theorem via abbreviations
Martin Klazar
https://arxiv.org/abs/2510.02620 https://arxiv.org/pdf/2510.02620

Formal Cantor's theorem via abbreviations
We devise a calculus of abbreviation schemes for ZF formulas. These formulas are certain words over the alphabet $$ \{x_1,\,x_2,\,\dots\}\cup \{\in,\,=,\,\neg, \,\to,\,\leftrightarrow,\,\wedge,\, \vee,\,\exists,\,\forall,\,(,\,)\} $$ and have interpretations in structures $M=\langle X,\in_M,=_M\rangle$. In them $X$ is any nonempty set, $\in_M\,\subset X\times X$ and $=_M$ is $\{\langle x,x\rangle\colon\;x\in X\}$. Using an abbreviation scheme of length 9 we produce a ZF sentence $φ$ with lengt…