Let C be a coalgebra over an arbitrary field K. We show that the study of the category C-Comod of left C-comodules reduces to the study of the category of (co)representations of a certain bicomodule, in case C is a bipartite coalgebra or a coradical square complete coalgebra, that is, C = C₁, the second term of the coradical filtration of C. If C = C₁, we associate with C a K-linear functor ${ℍ}_C:C-Comod\to H_C-Comod$ that restricts to a representation equivalence ${ℍ}_C:C-comod\to H_C-como{d}_{sp}^{•}$, where $H_C$ is a coradical square complete hereditary bipartite...

For a split graph order ℒ over a complete local regular domain $𝒪$ of dimension 2 the indecomposable Cohen-Macaulay modules decompose - up to irreducible projectives - into a union of the indecomposable Cohen-Macaulay modules over graph orders of type •—• . There, the Cohen-Macaulay modules filtered by irreducible Cohen-Macaulay modules are in bijection to the homomorphisms $\varphi :𝒪L^{\left(\mu \right)}\to 𝒪L^{\left(\nu \right)}$ under the bi-action of the groups $\left(Gl\right(\mu ,𝒪L),Gl(\nu ,𝒪L\left)\right)$, where $𝒪L=𝒪/〈\pi 〉$ for a prime π. This problem strongly depends on the nature of $𝒪L$. If $𝒪L$ is regular,...

We determine the Hochschild homology and cohomology of the generalized Weyl algebras of rank one which are of ‘quantum’ type in all but a few exceptional cases.

Let $A=K[a_1,...,a_n]$ be a (noncommutative) solvable polynomial algebra over a field $K$ in the sense of A. Kandri-Rody and V. Weispfenning [Non-commutative Gröbner bases in algebras of solvable type, J. Symbolic Comput. 9 (1990), 1–26]. This paper presents a comprehensive study on the computation of minimal free resolutions of modules over $A$ in the following two cases: (1) $A={⨁}_{p\in \mathbb{N}}{A}_p$ is an $\mathbb{N}$-graded algebra with the degree-0 homogeneous part $A_0=K$; (2) $A$ is an $\mathbb{N}$-filtered algebra with the filtration ${\left\{F_{p}A\right\}}_{p\in \mathbb{N}}$ determined by a positive-degree...

Let $K$ be a field and $S=K[x_1,...,x_m,y_1,...,y_n]$ be the standard bigraded polynomial ring over $K$. In this paper, we explicitly describe the structure of finitely generated bigraded “sequentially Cohen-Macaulay” $S$-modules with respect to $Q=(y_1,...,y_n)$. Next, we give a characterization of sequentially Cohen-Macaulay modules with respect to $Q$ in terms of local cohomology modules. Cohen-Macaulay modules that are sequentially Cohen-Macaulay with respect to $Q$ are considered.

A ring $R$ is called a right $\mathrm{PS}$-ring if its socle, $\mathrm{Soc}\left(R_R\right)$, is projective. Nicholson and Watters have shown that if $R$ is a right $\mathrm{PS}$-ring, then so are the polynomial ring $R\left[x\right]$ and power series ring $R\left[\right[x\left]\right]$. In this paper, it is proved that, under suitable conditions, if $R$ has a (flat) projective socle, then so does the skew inverse power series ring $R\left[[x^{-1};\alpha ,\delta ]\right]$ and the skew polynomial ring $R[x;\alpha ,\delta ]$, where $R$ is an associative ring equipped with an automorphism $\alpha$ and an $\alpha$-derivation $\delta$. Our results extend and unify many existing results....