Hi. I know, by the Bass-Papp theorem, that if every direct sum of injective $R$-modules is injective then $R$ is Noetherian. I would like to know if there exists a direct sum of injective $R$-modules injective with $R$ non-Noetherian. Of course if the sum is a finite sum of injective modules, then it is injective; so I assume that the sum is infinite (let's say that all the modules are nonzero).
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A module is called $\Sigma$-injective if a direct sum of arbitrarily (equivalently, countably infinitely) many copies of that module is injective. So it suffices to find an example of a $\Sigma$-injective module over a non-noetherian ring. Apart from silly examples such as a direct product of two rings one of which is one-sided noetherian and the other of which is not, the main theorem of C. Megibben, “Countable injective modules are $\Sigma$-injective,” Proc. Amer. Math. Soc. 84 (1982), no. 1, 8–10, says what the title indicates. This gives all sorts of examples of $\Sigma$-injective modules over non-noetherian rings. |
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