Cohen-Macaulay Representations by Graham J. Leuschke

By Graham J. Leuschke

This e-book is a accomplished remedy of the illustration conception of maximal Cohen-Macaulay (MCM) modules over neighborhood earrings. This subject is on the intersection of commutative algebra, singularity thought, and representations of teams and algebras. introductory chapters deal with the Krull-Remak-Schmidt Theorem on strong point of direct-sum decompositions and its failure for modules over neighborhood jewelry. Chapters 3-10 research the valuable challenge of classifying the jewelry with basically finitely many indecomposable MCM modules as much as isomorphism, i.e., jewelry of finite CM variety. the basic material--ADE/simple singularities, the double branched disguise, Auslander-Reiten concept, and the Brauer-Thrall conjectures--is lined essentially and fully. a lot of the content material hasn't ever earlier than seemed in ebook shape. Examples comprise the illustration thought of Artinian pairs and Burban-Drozd's similar development in measurement , an creation to the McKay correspondence from the viewpoint of maximal Cohen-Macaulay modules, Auslander-Buchweitz's MCM approximation thought, and a cautious remedy of nonzero attribute. the remainder seven chapters current effects on bounded and countable CM variety and at the illustration conception of completely reflexive modules.

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C r 1 ]tr ) equal to [ c 1 , . . , c r i ]tr . ) Let V be the k-subspace of W consisting of all elements ∂( u) + X ∂(v) + X 3 ∂( Hv) , as u and v run over C , where X = ( x, 0, . . , 0) and H is the nilpotent Jordan block with 1 on the superdiagonal and 0 elsewhere. (i) Prove that W is generated as a D -module by all elements of the form ∂( u), u ∈ C , so that in particular DV = W . (Hint: it suffices to consider elements w = (w1 , . . ) (ii) Prove that V →W is indecomposable along the same lines as the arguments in Chapter 4.

Proof. Let G be the subgroup of Z(t) generated by Λ, and write Z(t) /G = C 1 ⊕ · · · ⊕ C s , where each C i is a cyclic group. Then Z(t) /G can be embedded in (R/Z)(s) . 16. Since Z(t) /G embeds in Cl(B), there is a group homomorphism : Z(t) −→ Cl(B) with ker( ) = G . Let { e 1 , . . , e t } be the standard basis of Z(t) . For each i t, write ( e i ) = [L i ], where L i is a divisorial ideal of B representing the divisor class of ( e i ). Next we use Heitmann’s amazing theorem [Hei93], which implies that B is the completion of some local unique factorization domain R .

Now we apply (i) of the lemma to the first short exact sequence, to conclude that N is extended. 7 Lemma. Let (R, m) be a local ring with completion R , and let 0 −→ X −→ Y −→ Z −→ 0 be an exact sequence of finitely generated R -modules. (i) Assume X and Z are extended. g. if Z is locally free on the punctured spectrum of R ), then Y is extended. §2. Realization in dimension one (ii) Assume Y and Z are extended. g. if Z has finite length), then X is extended. (iii) Assume X and Y are extended. g.

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