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For questions on modules over rings.
9
votes
Accepted
Are there two non-isomorphic modules such that all the Hom-sets are isomorphic?
K. Bongartz, "A generalization of a theorem of Auslander":
Let R be a commutative ring and A an abelian R-linear category such
that each morphism set in A has finite length as an R-module. Let …
10
votes
Classification of pairs of commuting endomorphisms
The classification of $K[X,Y]$-modules of finite length is of wild type, meaning that a classification of these modules would contain within it a classification of all finite-dimensional $\Lambda$-modules … In fact, even the classification of those $K[X,Y]$-modules annihilated by $(X^2, XY^2,Y^3)$ is a wild problem, as shown by Drozd. …
4
votes
Accepted
Homological dimensions of module
See Fossum, Foxby, Griffith, and Reiten, "Minimal injective resolutions with applications to dualizing modules and Gorenstein modules" (Theorem 1.1) and also Roberts, "Two applications of dualizing complexes …
2
votes
when there is an injection $0 \to R \to K_R$?
As soon as the dimension of $R$ is at least $1$ and $R$ is locally Gorenstein at the associated primes, then $K$ is isomorphic to an ideal of pure height $1$. In particular $K$ (as an ideal) contains …
8
votes
Accepted
Extension problem
$\mathrm{Ext} = \mathrm{Ext}^1$ does not classify the middle terms up to isomorphism. It classifies the short exact sequences up to equivalence, where the equivalence relation is generated by commuta …