arXiv subjects

Carlo Klapproth

Publications and source records attributed to Carlo Klapproth.

7 recordsLinked to original sources

arXiv2024

On $n$-exact categories I: The existence and uniqueness of maximal $n$-exact structures

This paper is the first part of a series that investigates the existence of $n$-exact structures on idempotent complete additive categories for positive integers $n$. It is shown that every idempotent complete additive category has a unique maximal $n$-exact structure. We achieve this by constructing a bijection between $n$-exact structures on a category and certain subcategories of its functor category following ideas of Enomoto.

math.CT↗

arXiv2024

Obstructions to semiorthogonal decompositions for singular projective varieties II: Representation theory

We show that odd-dimensional projective varieties with tilting objects and only ADE-hypersurface singularities are nodal, i.e. they only have $A_1$-singularities. This is a very special case of more general obstructions to the existence of semiorthogonal decompositions for projective Gorenstein varieties. More precisely, for many isolated hypersurface singularities, we show that Kuznetsov-Shinder's categorical absorptions of singularities cannot contain tilting objects. The key idea is to compare singularity categories of projective varieties to singularity categories of finite-dimensional associative Gorenstein algebras. The former often contain special generators, called cluster-tilting objects, which typically have loops and $2$-cycles in their quivers. In contrast, quivers of cluster-tilting objects in the latter categories, can never have loops or $2$-cycles.

math.AG↗

arXiv2023

Idempotent completions of $n$-exangulated categories

Suppose $(\mathcal{C},\mathbb{E},\mathfrak{s})$ is an $n$-exangulated category. We show that the idempotent completion and the weak idempotent completion of $\mathcal{C}$ are again $n$-exangulated categories. Furthermore, we also show that the canonical inclusion functor of $\mathcal{C}$ into its (resp. weak) idempotent completion is $n$-exangulated and $2$-universal among $n$-exangulated functors from $(\mathcal{C},\mathbb{E},\mathfrak{s})$ to (resp. weakly) idempotent complete $n$-exangulated categories. Furthermore, we prove that if $(\mathcal{C},\mathbb{E},\mathfrak{s})$ is $n$-exact, then so too is its (resp. weak) idempotent completion. We note that our methods of proof differ substantially from the extriangulated and $(n+2)$-angulated cases. However, our constructions recover the known structures in the established cases up to $n$-exangulated isomorphism of $n$-exangulated categories.

math.CT↗

arXiv2023

$n$-Extension closed subcategories of $n$-exangulated categories

Let $n$ be a positive integer. We show that an $n$-extension closed subcategory of an $n$-exangulated category naturally inherits an $n$-exangulated structure through restriction of the ambient $n$-exangulated structure. Furthermore, we show that a strong version of the Obscure Axiom holds for $n$-exangulated categories, where $n \geq 2$. This allows us to characterize $n$-exact categories as $n$-exangulated categories with monic inflations and epic deflations. We also show that for an extriangulated category condition (WIC), which was introduced by Nakaoka and Palu, is equivalent to the underlying additive category being weakly idempotent complete. We then apply our results to show that $n$-extension closed subcategories of an $n$-exact category are again $n$-exact. Furthermore, we recover and improve results of Klapproth and Zhou.

math.RT↗

arXiv2023

When does the Auslander-Reiten translation operate linearly on the Grothendieck group? -- Part I

For a hereditary, finite-dimensional algebra $A$ the Coxeter transformation extends the action of the Auslander--Reiten translation on the non-projective indecomposable modules to a linear endomorphism of the Grothendieck group of the category of finitely generated $A$-modules. It is natural to ask whether other algebras admit a similar linear extension. We show that this is indeed the case for all Nakayama algebras. Conversely, we show that finite-dimensional algebras with non-acyclic and connected quiver admitting such a linear extension are already cyclic Nakayama algebras.

math.RT↗

arXiv2023

A method for constructing minimal projective resolutions over idempotent subrings

We show how to obtain minimal projective resolutions of finitely generated modules over an idempotent subring $Γ_e := (1-e)R(1-e)$ of a semiperfect noetherian basic ring $R$ by a construction inside $\mathsf{mod} R$. This is then applied to investigate homological properties of idempotent subrings $Γ_e$ under the assumption of $R/\langle 1-e\rangle$ being a right artinian ring. In particular, we prove the conjecture by Ingalls and Paquette that a simple module $S_e := eR /\operatorname{rad} eR$ with $\operatorname{Ext}_R^1(S_e,S_e) = 0$ is self-orthogonal, that is $\operatorname{Ext}^k_R(S_e,S_e)$ vanishes for all $k \geq 1$, whenever $\operatorname{gl} R$ and $\operatorname{pdim} eR(1-e)_{Γ_e}$ are finite. Indeed, a slightly more general result is established, which applies to sandwiched idempotent subrings: Suppose $e \in R$ is an idempotent such that all idempotent subrings $Γ$ sandwiched between $Γ_e$ and $R$, that is $Γ_e \subset Γ\subset R$, have finite global dimension. Then the simple summands of $S_e$ can be numbered $S_1, \dots, S_n$ such that $\operatorname{Ext}_R^k(S_i, S_j) = 0$ for $1 \leq j \leq i \leq n$ and all $k > 0$.

math.RT↗

arXiv2021

$n$-Exact categories arising from $(n+2)$-angulated categories

Let $\mathscr{F}$ be an $(n+2)$-angulated Krull-Schmidt category and $\mathscr{A} \subset \mathscr{F}$ an $n$-extension closed, additive and full subcategory with $\operatorname{Hom}_{\mathscr{F}}(Σ_n \mathscr{A}, \mathscr{A}) = 0$. Then $\mathscr{A}$ naturally carries the structure of an $n$-exact category in the sense of Jasso, arising from short $(n+2)$-angles in $\mathscr{F}$ with objects in $\mathscr{A}$ and there is a binatural and bilinear isomorphism $\operatorname{YExt}^{n}_{(\mathscr{A},\mathscr{E}_{\mathscr{A}})}(A_{n+1},A_0) \cong \operatorname{Hom}_{\mathscr{F}}(A_{n+1}, Σ_n A_{0})$ for $A_0, A_{n+1} \in \mathscr{A}$. For $n = 1$ this has been shown by Dyer and we generalize this result to the case $n > 1$. On the journey to this result, we also develop a technique for harvesting information from the higher octahedral axiom (N4*) as defined by Bergh and Thaule. Additionally, we show that the axiom (F3) for pre-$(n+2)$-angulated categories, introduced by Geiss, Keller and Oppermann and stating that a commutative square can be extended to a morphism of $(n+2)$-angles, implies a stronger version of itself.

math.RT↗