JournalsjemsVol. 16, No. 9pp. 1775–1816

Prescribing endomorphism algebras of n\aleph_n-free modules

  • Rüdiger Göbel

    Universität Duisburg-Essen, Germany
  • Daniel Herden

    Universität Duisburg-Essen, Germany
  • Saharon Shelah

    The Hebrew University of Jerusalem, Israel
Prescribing endomorphism algebras of $\aleph_n$-free modules cover
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Abstract

It is a well-known fact that modules over a commutative ring in general cannot be classified, and it is also well-known that we have to impose severe restrictions on either the ring or on the class of modules to solve this problem. One of the restrictions on the modules comes from freeness assumptions which have been intensively studied in recent decades. Two interesting, distinct but typical examples are the papers by Blass and Eklof, both jointly with Shelah. In the first case the authors consider almost-free abelian groups and assume the existence of large canonical, free subgroups. Nevertheless, there exist 1\aleph_1-separable torsion-free groups GG of size 1\aleph_1 with a basic subgroup BB of rank 1\aleph_1 such that all subgroups of GG disjoint from BB are also free, but the groups GG are still not free. What else can we say about GG? The other paper deals with Kaplansky's test problems (which are excellent indicators that the objects defy classification). The authors are able to construct very free abelian groups and verify the test problems for them by a careful choice of \emph{particular} elements of their endomorphism rings.

Accordingly, we want to investigate and construct n\aleph_n-free RR-modules MM (with nn an arbitrary, but fixed natural number) over a domain RR with EndRM=R_RM=R for the first time more systematically and uniformly. Recall that MM is n\aleph_n-free, if every subset of size <n<\aleph_n is contained in a pure, free submodule of MM. The requirement EndRM=R_RM=R implies that MM is indecomposable, hence complicated. (We will also allow that EndRM_RM is a prescribed RR-algebra, as in the title of this paper.)

By now it is folklore to construct such modules MM using additional set theoretic axioms, most notably Jensen's \diamondsuit-principle. In this case the freeness-condition can even be strengthened, see [6] and many examples in [9]. However, if we insist on proving this result in ordinary ZFC, then the known arguments fail: The classical constructions from the fundamental paper by Corner [2] do not apply because they are based on pure submodules of pp-adic completions of free AA-modules, which are never even 1\aleph_1-free. If we use Shelah's Black Box instead of Jensen's \diamondsuit-principle, then the constructed modules MM are still 1\aleph_1-free, but always fail to be even 2\aleph_2-free, see [4]. Thus we must develop new methods, which are presented for the first time in Sections 2 to 6, to achieve the desired result (Main Theorem 7.6). With these methods we provide a useful tool for a wide range of problems concerning n\aleph_n-free structures which can then be attacked.

Cite this article

Rüdiger Göbel, Daniel Herden, Saharon Shelah, Prescribing endomorphism algebras of n\aleph_n-free modules. J. Eur. Math. Soc. 16 (2014), no. 9, pp. 1775–1816

DOI 10.4171/JEMS/475