Almost free splitters
Tom 81 / 1999
Let R be a subring of the rationals. We want to investigate self splitting R-modules G, that is, such that $Ext_R(G,G) = 0$. For simplicity we will call such modules splitters (see ). Also other names like stones are used (see a dictionary in Ringel's paper ). Our investigation continues . In  we answered an open problem by constructing a large class of splitters. Classical splitters are free modules and torsion-free, algebraically compact ones. In  we concentrated on splitters which are larger than the continuum and such that countable submodules are not necessarily free. The "opposite" case of $ℵ_1$-free splitters of cardinality less than or equal to $ℵ_1$ was singled out because of basically different techniques. This is the target of the present paper. If the splitter is countable, then it must be free over some subring of the rationals by Hausen . In contrast to the results of  and in accordance with  we can show that all $ℵ_1$-free splitters of cardinality $ℵ_1$ are free indeed.