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Mathematics Research Report MRR95-077

Homomorphisms In Higher Complexity Quotient Categories

Jon F. Carlson and Wayne W. Wheeler

Abstract: In previous work the authors together with Peter Donovan initiated a study of complexity quotient categories for the modular group algebra of a finite group G. Let Mc denote the full subcategory of the stable category consisting of all finitely generated modules having complexity at most c. The previous work studied quotients of the form Mc/Mc-1; the current paper is devoted to studying quotients Mc/Md when c-d\ge 2. The main tool used here, a chain complex called the modular Koszul complex, gives rise to a spectral sequence that provides a description of homomorphisms in these higher complexity quotient categories. This description is then used to decompose the endomorphism ring of the trivial module by producing a complete set of primitive orthogonal idempotents determined by certain subvarieties of VG(k). The paper also considers connections with Cech cohomology and with the idempotent modules studied by Rickard.


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