Hochschild cohomology via incidence algebras

Given an algebra A we associate an incidence algebra A(\Sigma) and compare their Hochschild cohomology groups.Comment: 16 pages, Section 3.2 deleted, Section 4 adde

Cartan-Leray spectral sequence for Galois coverings of linear categories

We provide a Cartan-Leray type spectral sequence for the Hochschild-Mitchell (co)homology of a Galois covering of linear categories. We infer results relating the Galois group and Hochschild cohomology in degree one

Cohomology of the Grothendieck construction

We consider cohomology of small categories with coefficients in a natural system in the sense of Baues and Wirsching. For any funtor L: K -> CAT, we construct a spectral sequence abutting to the cohomology of the Grothendieck construction of L in terms of the cohomology of K and of L(k), for k an object in K.Comment: 13 page

On the first Hochschild cohomology group of a cluster-tilted algebra

Given a cluster-tilted algebra B, we study its first Hochschild cohomology group HH^1(B) with coefficients in the B-B-bimodule B. If C is a tilted algebra such that B is the relation extension of C, then we show that if C is constrained, or else if B is tame, then HH^1(B) is isomorphic, as a k-vector space, to the direct sum of HH^1(C) with k^{n\_{B,C}}, where n\_{B,C} is an invariant linking the bound quivers of B and C. In the representation-finite case, HH^1(B) can be read off simply by looking at the quiver of B.Comment: 30 page

On universal gradings, versal gradings and Schurian generated categories

Categories over a field $k$ can be graded by different groups in a connected way; we consider morphisms between these gradings in order to define the fundamental grading group. We prove that this group is isomorphic to the fundamental group \`a la Grothendieck as considered in previous papers. In case the $k$-category is Schurian generated we prove that a universal grading exists. Examples of non Schurian generated categories with universal grading, versal grading or none of them are considered.Comment: Final version to appear in the Journal of Noncommutative Geometry, 21 page

Fundamental group of Schurian categories and the Hurewicz isomorphism

Let k be a field. We attach a CW-complex to any Schurian k-category and we prove that the fundamental group of this CW-complex is isomorphic to the intrinsic fundamental group of the k-category. This extends previous results by J.C. Bustamante. We also prove that the Hurewicz morphism from the vector space of abelian characters of the fundamental group to the first Hochschild-Mitchell cohomology vector space of the category is an isomorphism.Comment: Final version to appear in Documenta Mathematica, 14 page