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Examples of non-finitely generated projective modules. (English) Zbl 0806.16002

Abrams, Gene (ed.) et al., Methods in module theory. Papers presented at the conference, held in Colorado Springs, CO, USA, May 29-June 1, 1991. New York: Marcel Dekker, Inc. Lect. Notes Pure Appl. Math. 140, 271-278 (1993).
All rings considered are associative with identity. All modules are left unital. An \(R\)-module \(M\) is said to be NFG if it is not finitely generated. The author presents for a particular ring \(R\) three new examples of unusual NFG projective \(R\)-modules: Example 1. There exist projective \(R\)-modules \(P\), \(Q\) and \(P\cap Q\) such that \(P \times Q \cong (P\cap Q) \times R\) where \(P\), \(Q\) and \(P \cap Q\) are NFG, indecomposable, and pairwise non-isomorphic; Example 2. There exists a nonzero projective \(R\)-module \(M\) such that each nonzero direct summand of \(M\) is NFG and decomposable; Example 3. There exists a projective \(R\)- module \(M\) such that each nonzero direct summand of \(M\) is NFG but, if \(M = A \oplus B\), one summand is a finite direct sum of indecomposable submodules and the other summand is decomposable.
For the entire collection see [Zbl 0772.00017].

MSC:

16D40 Free, projective, and flat modules and ideals in associative algebras
13C10 Projective and free modules and ideals in commutative rings