Semi-rigid classes of cotorsion-free abelian groups

Cotorsion theories cogenerated by א1-free abelian groups

Given an א1-free abelian group G we characterize the class CG of all torsion abelian groups T satisfying Ext(G, T ) = 0 assuming the special continuum hypothesis CH. Moreover, in Gödel’s constructable universe we prove that this characterizes CG for arbitrary torsion-free abelian G. It follows that there exist some ugly א1-free abelian groups.

G.C.H. implies existence of many rigid almost free abelian groups

This result has been generalized in various directions, e.g. replacing the ground ring Z by a more general ring and dropping the cardinal restriction (in Corner’s result this is actually |A| < 20), see [2], [20], [8], [9], [4], [23]. Other extensions include abelian groups which are not necessarily torsionfree, see Corner, Göbel [4] for references. Here we are interested in a different strength...

some special classes of n-abelian groups

‎given an integer $n$‎, ‎we denote by $mathfrak b_n$ and $mathfrak c_n$ the classes of all groups $g$ for which the map $phi_{n}:gmapsto g^n$ is a monomorphism and an epimorphism of $g$‎, ‎respectively‎. ‎in this paper we give a characterization for groups in $mathfrak b_n$ and for groups in $mathfrak c_n$‎. ‎we also obtain an arithmetic description of the set of all integers $n$ such that a gr...

Intuitionistic Free Abelian Groups

Free abelian lattice-ordered groups

Let n be a positive integer and FA`(n) be the free abelian latticeordered group on n generators. We prove that FA`(m) and FA`(n) do not satisfy the same first-order sentences in the language L={+,−, 0,∧,∨} if m 6= n. We also show that Th(FA`(n)) is decidable iff n ∈ {1, 2}. Finally, we apply a similar analysis and get analogous results for the free finitely generated vector lattices. A. M. S. C...