93 research outputs found
Power series rings and projectivity
We show that a formal power series ring over a noetherian ring
is not a projective module unless is artinian. However, if is local, then behaves like a projective module in the sense that
for all -adically complete -modules.
The latter result is shown more generally for any flat -module instead
of . We apply the results to the (analytic) Hochschild cohomology over
complete noetherian rings.Comment: Mainly thanks to remarks and pointers by L.L.Avramov and S.Iyengar,
we added further context and references. To appear in Manuscripta
Mathematica. 7 page
Homology of perfect complexes
It is proved that the sum of the Loewy lengths of the homology modules of a
finite free complex F over a local ring R is bounded below by a number
depending only on R. This result uncovers, in the structure of modules of
finite projective dimension, obstructions to realizing R as a closed fiber of
some flat local homomorphism. Other applications include, as special cases,
uniform proofs of known results on free actions of elementary abelian groups
and of tori on finite CW complexes. The arguments use numerical invariants of
objects in general triangulated categories, introduced here and called levels.
They allow one to track, through changes of triangulated categories,
homological invariants like projective dimension, as well as structural
invariants like Loewy length. An intermediate result sharpens, with a new
proof, the New Intersection Theorem for commutative algebras over fields. Under
additional hypotheses on the ring stronger estimates are proved for Loewy
lengths of modules of finite projective dimension.Comment: This version corrects an error in the statement (and proof) of
Theorem 7.4 in the published version of the paper [Adv. Math. 223 (2010)
1731--1781]. These changes do not affect any other results or proofs in the
paper. A corrigendum has been submitted
On computation of the first Baues--Wirsching cohomology of a freely-generated small category
The Baues--Wirsching cohomology is one of the cohomologies of a small
category. Our aim is to describe the first Baues--Wirsching cohomology of the
small category generated by a finite quiver freely. We consider the case where
the coefficient is a natural system obtained by the composition of a functor
and the target functor. We give an algorithm to obtain generators of the vector
space of inner derivations. It is known that there exists a surjection from the
vector space of derivations of the small category to the first Baues--Wirsching
cohomology whose kernel is the vector space of inner derivations.Comment: 11 page
A McKay correspondence for reflection groups
We construct a noncommutative desingularization of the discriminant of a finite reflection group G as a quotient of the skew group ring A=S∗G. If G is generated by order 2 reflections, then this quotient identifies with the endomorphism ring of the reflection arrangement A(G) viewed as a module over the coordinate ring SG/(Δ) of the discriminant of G. This yields, in particular, a correspondence between the nontrivial irreducible representations of G to certain maximal Cohen–Macaulay modules over the coordinate ring SG/(Δ). These maximal Cohen–Macaulay modules are precisely the nonisomorphic direct summands of the coordinate ring of the reflection arrangement A(G) viewed as a module over SG/(Δ). We identify some of the corresponding matrix factorizations, namely, the so-called logarithmic (co-)residues of the discriminant
Noncommutative resolutions of discriminants
We give an introduction to the McKay correspondence and its connection to
quotients of by finite reflection groups. This yields a natural
construction of noncommutative resolutions of the discriminants of these
reflection groups. This paper is an extended version of E.F.'s talk with the
same title delivered at the ICRA.Comment: 15 pages, 4 figures. Final version to appear in Contemporary
Mathematics 705, "Representations of Algebras
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