Subgroups of the Baer-Specker Group with Few Endomorphisms but Large Dual

Almost disjoint pure subgroups of the Baer-Specker group

We prove in ZFC that the Baer-Specker group Z^omega has 2^{aleph_1} non-free pure subgroups of cardinality aleph_1 which are almost disjoint: there is no non-free subgroup embeddable in any pair.

Strong subgroup chains and the Baer-Specker group

Examples are given of non-elementary properties that are preserved under C-filtrations for various classes C of Abelian groups. The Baer-Specker group is never the union of a chain of proper subgroups with cotorsionfree quotients. Cotorsion-free groups form an abstract elementary class (AEC). The Kaplansky invariants of the Baer-Specker group are used to determine the AECs defined by the perps of the Baer-Specker quotient groups that are obtained by factoring the Baer-Specker group B of a ZFC extension by the Baer-Specker group A of the ground model, under various hypotheses, yielding informat…

The combinatorics of the Baer-Specker group

Denote the integers by Z and the positive integers by N. The groups Z^k (k a natural number) are discrete, and the classification up to isomorphism of their (topological) subgroups is trivial. But already for the countably infinite power Z^N of Z, the situation is different. Here the product topology is nontrivial, and the subgroups of Z^N make a rich source of examples of non-isomorphic topological groups. Z^N is the Baer-Specker group. We study subgroups of the Baer-Specker group which possess group theoretic properties analogous to properties introduced by Menger (1924), Hurewicz (1925), Ro…

Reflexive subgroups of the Baer-Specker group and Martin's axiom

In two recent papers (math.LO/0003164 and math.LO/0003165) we answered a question raised in the book by Eklof and Mekler (p. 455, Problem 12) under the set theoretical hypothesis of diamondsuit_{aleph_1} which holds in many models of set theory, respectively of the special continuum hypothesis (CH). The objects are reflexive modules over countable principal ideal domains R, which are not fields. Following H.Bass, an R-module G is reflexive if the evaluation map s:G ---> G^{**} is an isomorphism. Here G^*=Hom(G,R) denotes the dual module of G. We proved the existence of reflexive R-modules G of…

将环表示为阿贝尔群的自同态环

Representing a ring as the endomorphism ring of an abelian group

本文在ZFC中通过结合κ-框架与Shelah超黑箱，构造了自同态环同构于给定环的μ^+-自由模，部分解决了Göbel和Trlifaj关于环表示的问题。

ZFC中的华莱士问题与可数紧无挠阿贝尔群

The Wallace problem and countably compact torsion-free Abelian groups in ZFC

该研究在ZFC公理体系下，证明基数为$\mathfrak c$的无挠阿贝尔群可赋予无非平凡收敛序列的豪斯多夫可数紧群拓扑，构造出满足双侧消去律但非群的可数紧拓扑半群，否定回答了华莱士问题，并衍生出相关拓扑结构结论。