Saharon Shelah
We investigate the Whiteheadness of Borel abelian groups (aleph_1-free, wlog, as otherwise this is trivial). We show that CH (and even WCH) implies any such abelian group is free, and always aleph_2-free.
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As a consequence of identifying the principle described in the title, we prove that for any uncountable cardinal lambda, if there is a lambda-free Whitehead group of cardinality lambda which is not free, then there are many ``nice'' Whitehead groups of cardinality lambda which are not free.
We show that it is consistent that there is a strongly aleph_1-free aleph_1-coseparable group of cardinality aleph_1 which is not aleph_1-separable.
The connections between Whitehead groups and uniformization properties were investigated by the third author in [Sh:98]. In particular it was essentially shown there that there is a non-free Whitehead (respectively, aleph_1-coseparable) group of cardinality aleph_1 if and only if there is a ladder system on a stationary subset of omega_1 which satisfies 2-uniformization (respectively, omega-uniformization). These techniques allowed also the proof of various independence and consistency results about Whitehead groups, for example that it is consistent that there is a non-free Whitehead group of…
We consider the question of when the dual of a Whitehead group is a test group for Whitehead groups. This turns out to be equivalent to the question of when the tensor product of two Whitehead groups is Whitehead. We investigate what happens in different models of set theory.
We prove that it is consistent that there is a non-reflexive Whitehead group, in fact one whose dual group is free. We also prove that it is consistent that there is a group A such that Ext(A,Z) is torsion and Hom(A,Z)=0. As an application we show the consistency of the existence of new co-Moore spaces.
Assume G.C.H. and kappa is the first uncountable cardinal such that there is a kappa-free abelian group which is not a Whitehead (abelian) group. We prove that kappa is necessarily an inaccessible cardinal
The Whitney group $K_1(A)$ is isomorphic to $A^\times \times \operatorname{SK}_1(A)$ for some subgroup $\operatorname{SK}_1(A)$, where $A$ is a commutative ring and $A^\times$ denotes the set of units in $A$. Consider an o-minimal expansion of a real closed field $\mathcal R=(R,0,1,+,\cdot,\ldots)$. Let $M$ be an affine definable $C^r$ manifold, where $r$ is a nonnegative integer. We demonstrate its homotopy theorem and that the group $\operatorname{SK}_1(C_{\text{df}}^r)$ is isomorphic to $\operatorname{SK}_1(C_{\text{df}}^0(M))$, where $C_{\text{df}}^r(M)$ denotes the ring of definable $C^r$…
Computable Scott sentences and the weak Whitehead problem for finitely presented groups
本文证明可计算Hopfian有限展示结构具有可计算d-Σ₂ Scott语句当且仅当其弱Whitehead问题可判定，并据此推出双曲群与polycyclic-by-finite群均具有此类语句，同时证明弱Hopfian有限展示群由其∃⁺-types强定义。
Between Whitehead groups and uniformization
本文在ZFC中证明了关于可数序数平稳集的梯子系统一致化与强自由阿贝尔群的余可分性等价，解决了Eklof和Mekler专著中的问题B3和B4。
Condensed Whitehead Problem
针对独立于ZFC的怀特海问题，研究者在浓缩数学中给出类似问题的肯定解答，并证明大小为κ的阿贝尔群A是自由的等价于特定Ext函子取值为0。
Whitehead's problem and condensed mathematics
本文探讨了Whitehead问题在压缩数学中的独立性，并展示了其在不同范畴下的不同解法及一致性问题。
We give a combinatorial equivalent to the existence of a non-free hereditarily separable group of cardinality aleph_1. This can be used, together with a known combinatorial equivalent of the existence of a non-free Whitehead group, to prove that it is consistent that every Whitehead group is free but not every hereditarily separable group is free. We also show that the fact that Z is a p.i.d. with infinitely many primes is essential for this result.
We show that Vizing's Theorem holds in the Borel context for graphs induced by actions of 2-ended groups, and ask whether it holds more generally for everywhere two ended Borel graphs.
Borel Edge Colorings for Finite Dimensional Groups
本文研究了Borel渐近维数在生成Schreier图的Borel群作用下的边着色应用，发现其能恢复Vizing界并精确确定自由交换群作用的Borel边色数。
We construct pairs of marked groups with isomorphic Cayley graphs but different Borel chromatic numbers for the free parts of their shift graphs. This answers a question of Kechris and Marks. We also show that these graphs have different Baire measurable and measure chromatic numbers, answering analogous versions of the question.
We study Borel equivalence relations induced by Borel actions of tsi Polish groups on standard Borel spaces. We characterize when such an equivalence relation admits classification by countable structures using a variant of the $\mathbb G_0$-dichotomy. In particular, we find a class that serves as a base for non-classification by countable structures for these equivalence relations under Borel reducibility. We use this characterization together with the result of [B. D. Miller, to appear in the Journal of Mathematical Logic] to show that if such an equivalence relation admits classification by…
A Borel maximal cofinitary group
本文构造了一个 Borel 极大余有限群，解决了描述集合论与群论交叉领域中的构造性问题。
A proof of the Borel completeness of torsion free abelian groups is presented. This proof differs considerably from the approach of Paolini-Shelah.
We prove that the Borel space of torsion-free Abelian groups with domain $ω$ is Borel complete, i.e., the isomorphism relation on this Borel space is as complicated as possible, as an isomorphism relation. This solves a long-standing open problem in descriptive set theory, which dates back to the seminal paper on Borel reducibility of Friedman and Stanley from 1989.
In this paper, we show that Frucht's theorem holds in Borel setting. More specifically, we prove that any standard Borel group can be realized as the Borel automorphism group of a Borel graph. A slight modification of our construction also yields the following result in topological setting: Any Polish group can be realized as the homeomorphic automorphism group of a $\mathbf{Δ^0_2}$-graph on a Polish space.
We study topological groups that can be defined as Polish, pro-countable abelian groups, as non-archimedean abelian groups or as quasi-countable abelian groups, i.e., Polish subdirect products of countable, discrete groups, endowed with the product topology. We characterize tame groups in this class, i.e., groups such that all orbit equivalence relations induced by their continuous actions on Polish spaces are Borel, and relatively tame groups $G$, i.e., groups such that every diagonal action $α\times β$ of $G$ induces a Borel orbit equivalence relation, provided that the actions $α$, $β$ of $…
All Borel Group Extensions of Finite-Dimensional Real Space Are Trivial
本文结合同调代数与描述集合论，证明对所有N≥2，有限维实空间加法群通过可数阿贝尔Borel群的扩张均为平凡，完善了Kanovei和Reeken关于N=1的早期结果。
We study global dynamical properties of the isometry group of the Borel randomization of a separable complete structure. In particular, we show that if properties such as the Rohklin property, topometric generics, extreme amenability hold for the isometry group of the structure, they also hold in the isometry group of the randomization.
We show that a {\it Borel} action of a Polish group on a standard Borel space is Borel isomorphic to a {\it continuous} action of the group on a Polish space, and we apply this result to three aspects of the theory of Borel actions of Polish groups: universal actions, invariant probability measures, and the Topological Vaught Conjecture. We establish the existence of universal actions for any given Polish group, extending a result of Mackey and Varadarajan for the locally compact case. We prove an analog of Tarski's theorem on paradoxical decompositions by showing that the existence of an inva…
Componentwise Polish groupoids and equivalence relations
本文研究带分量式波兰拓扑的Borel群胚，证明满足适当公理时其Borel等价于波兰群作用的作用群胚，并将结果推广到拟波兰情形及若干经典工具。
We show that the enveloping space $X_G$ of a partial action of a Polish group $G$ on a Polish space $X$ is a standard Borel space, that is to say, there is a topology $τ$ on $X_G$ such that $(X_G, τ)$ is Polish and the quotient Borel structure on $X_G$ is equal to $Borel(X_G,τ)$. To prove this result we show a generalization of a theorem of Burgess about Borel selectors for the orbit equivalence relation induced by a group action and also show that some properties of the Vaught's transform are valid for partial actions of groups.
Non-Archimedean TSI Polish groups and their potential Borel complexity spectrum
本文将 Scott 分析变体应用于非阿基米德 TSI 波兰群的连续作用，研究了其潜在 Borel 复杂度谱，定义了通用轨道等价关系，识别了可分类性障碍，并推广了 Ding 和 Gao 的等价关系刻画结果。
Torsion-free abelian groups are faithfully Borel complete and pure embeddability is a complete analytic quasi-order
本文证明了无 torsion 的 Abel 群是忠实 Borel 完整的，并展示了纯可嵌入性是一个完全的解析准序。
We show that it is consistent with ordinary set theory ZFC and the generalized continuum hypothesis that there exist two separable abelian groups of cardinality aleph_1 which are filtration equivalent and one is a Whitehead group but the other is not. This solves one of the open problems of Eklof and Mekler.
We observe that Hall's free projective extension $P \mapsto F(P)$ of partial planes is a Borel map, and use a modification of the construction introduced in [9] to conclude that the class of countable non-Desarguesian projective planes is Borel complete. In the process, we also rediscover the main result of [7] on the realizability of every group as the group of collineations of some projective plane. Finally, we use classical results of projective geometry to prove that the class of countable Pappian projective planes is Borel complete.
Definable expansions on countable groups and countable Borel equivalence relations
本文在描述组合学框架下研究可数 Borel 等价关系与可数群的可定义扩张，建立二者对应及测度、范畴结构定理，并通过生成树、Ramsey 单色集和偏序线性化等示例说明。
On the Borel completeness of torsion-free abelian groups and a Slaman-Wehner degree spectrum
本文给出无挠阿贝尔群Borel完备性的简化证明，并构造一个无计算副本但可从所有非计算集计算的群，解决Downey-Goncharov问题。
Borel graphable equivalence relations
本文研究了哪些分析等价关系是Borel图化的，并探讨了图群及其结构性质。
A long standing open problem in the theory of hyperfinite equivalence relations asks if the orbit equivalence relation generated by a Borel action of a countable amenable group is hyperfinite. In this paper we prove that this question always has a positive answer when the acting group is polycyclic, and we obtain a positive answer for all free actions of a large class of groups including the lamplighter group and all virtually solvable groups having finite Prüfer rank. This marks the first time that a group of exponential volume-growth has been verified to have this property. In obtaining this…
We show that for any infinite countable group $G$ and for any free Borel action $G \curvearrowright X$ there exists an equivariant class-bijective Borel map from $X$ to the free part $\mathrm{Free}(2^G)$ of the $2$-shift $G \curvearrowright 2^G$. This implies that any Borel structurability which holds for the equivalence relation generated by $G \curvearrowright \mathrm{Free}(2^G)$ must hold a fortiori for all equivalence relations coming from free Borel actions of $G$. A related consequence is that the Borel chromatic number of $\mathrm{Free}(2^G)$ is the maximum among Borel chromatic numbers…
On the Borel complexity of the space of left-orderings of nilpotent groups
本文首次构造非交换左序群，其序空间共轭轨道关系有无穷轨道但Borel光滑，并给出幂零群光滑性的充分条件，同时证明幂零性非充分条件。
Hyperfiniteness and Borel asymptotic dimension of boundary actions of hyperbolic groups
本文为有限生成双曲群 Gromov 边界作用的轨道等价关系超有限性定理提供新的简短证明，并进一步证明此类作用具有有限 Borel 渐近维数。
Structural, point-free, non-Hausdorff topological realization of Borel groupoid actions
本文推广Becker--Kechris拓扑实现定理至群胚作用，基于范畴量词的代数性质给出新证明，适用于非Hausdorff及无点上下文，并刻画潜在开关系与一阶结构实现。
We study groups which satisfy Gardner's equidecomposition conjecture for uniformly distributed sets. We prove that an amenable group has this property if and only if it does not admit $(\mathbb{Z}/2\mathbb{Z}) *(\mathbb{Z}/2\mathbb{Z})$ as a quotient by a finite subgroup. Our technical contribution is an algorithm for rounding Borel flows for actions of amenable groups.
We show that a Borel action of a standard Borel group which is isomorphic to a sum of a countable abelian group with a countable sum of real lines and circles induces an orbit equivalence relation which is hypersmooth, i.e., Borel reducible to eventual agreement on sequences of reals, and it follows from this result along with the structure theory for locally compact abelian groups that Borel actions of Polish LCA groups induce orbit equivalence relations which are essentially hyperfinite, extending a result of Gao and Jackson and answering a question of Ding and Gao.
Let $Γ$ be a countable group and let $G$ be the Schreier graph of the free part of the Bernoulli shift of $Γ$ (with respect to some finite subset $F \subseteq Γ$). We show that the Borel fractional chromatic number of $G$ is equal to $1$ over the measurable independence number of $G$. As a consequence, we asymptotically determine the Borel fractional chromatic number of $G$ when $Γ$ is the free group, answering a question of Meehan.
Applications of Borel-definable homological algebra to locally compact groups
该研究将Borel可定义同调代数应用于局部紧群领域，刻画$\boldsymbol{\text{LCPAb}}$左心并证明多个相关范畴的左心无非零内射对象，推进了局部紧群同调理论的研究。
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