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On finitely generated projective modules and exchange rings. (English) Zbl 1023.16002

\(R\) denotes an associative ring with identity and all modules are right unital. Finitely generated projective modules (\(\text{FP}(R)\)) over an exchange ring \(R\) are studied first and it is shown that a module \(P\) is isomorphic to a direct summand of a module \(Q\) (resp. \(P\simeq Q\)) if and only if \(P/PI\) is isomorphic to a direct summand of \(Q/QI\) (resp. \(P/PI\simeq Q/QI\)) for any ideal \(I\) of \(R\) such that \(R/I\) is indecomposable and \(J\)-semisimple.
On the other hand, if \(R/J(R)\) is an exchange ring, then the cancellation law holds in \(\text{FP}(R)\) if and only if the cancellation law holds in \(\text{FP}(R/I)\) for any ideal \(I\) of \(R\) such that \(R/I\) is indecomposable and \(J\)-semisimple.
Properties of exchange rings \(R\) are studied next and results with respect to the unit 1-stable range condition and idempotents of \(R\) are obtained.

MSC:

16D40 Free, projective, and flat modules and ideals in associative algebras
16D70 Structure and classification for modules, bimodules and ideals (except as in 16Gxx), direct sum decomposition and cancellation in associative algebras)
16E50 von Neumann regular rings and generalizations (associative algebraic aspects)
19B10 Stable range conditions