arXiv subjects

Sebastian Opper

Publications and source records attributed to Sebastian Opper.

8 recordsLinked to original sources

arXiv2026

Hochschild cohomology of graded gentle algebras and intrinsic formality

We describe the (bigraded) Hochschild cohomology of graded gentle algebras along with the Gerstenhaber bracket and cup product. In particular, this yields a description of the Hochschild cohomology of partially wrapped Fukaya categories of surfaces in the sense of Haiden-Katzarkov-Kontsevich which have at least one stop. Our results are an important ingredient in the author's recent description of the derived Picard group of partially wrapped Fukaya categories and graded gentle algebras. As another application we provide a characterisation of intrinsically formal graded gentle algebras under mild assumptions.

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arXiv2025

Autoequivalences of Fukaya categories of surfaces and graded gentle algebras

We compute the derived Picard groups of partially wrapped Fukaya categories of surfaces in the sense of Haiden-Katzarkov-Kontsevich and the related graded gentle algebras. This includes the wrapped cases as introduced by Bocklandt. An important ingredient for our proof in characteristic zero is the exponential map from Hochschild cohomology to the derived Picard group introduced in recent work by the author. In positive characteristics, we combine deformation theory and formality results for Hochschild complexes to prove our results. Along the way we show that the surface together with its decorations forms a complete derived invariant of partially wrapped Fukaya categories and we prove analogous results for graded gentle algebras. This removes all previous restrictions from earlier results of this kind.

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arXiv2025

Reflexive dg categories in algebra and topology

Reflexive dg categories were introduced by Kuznetsov and Shinder to abstract the duality between bounded and perfect derived categories. In particular this duality relates their Hochschild cohomologies, autoequivalence groups, and semiorthogonal decompositions. We establish reflexivity in a variety of settings including affine schemes, simple-minded collections, chain and cochain dg algebras of topological spaces, Ginzburg dg algebras, and Fukaya categories of cotangent bundles and surfaces as well as the closely related class of graded gentle algebras. Our proofs are based on the interplay of reflexivity with gluings, derived completions, and Koszul duality. In particular we show that for certain (co)connective dg algebras, reflexivity is equivalent to derived completeness.

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arXiv2024

Integration of Hochschild cohomology, derived Picard groups and uniqueness of lifts

The paper introduces a partial integration map from the first Hochschild cohomology of any cohomologically unital A-infinity category over a field of characteristic zero to its derived Picard group. We discuss useful properties such as injectivity, naturality and the relation with the Baker-Campbell-Hausdorff formula. Based on the image of the integration map we propose a candidate for the identity component of the derived Picard group in the case of finite-dimensional graded algebras. As a first application of the integration map it is shown that the vanishing of its domain is a necessary condition for the uniqueness of lifts of equivalences from the homotopy category to the A-infinity-level. The final part contains applications to derived Picard groups of wrapped and compact Fukaya categories of cotangent bundles and their plumbings and an outlook on applications to derived Picard groups of partially wrapped Fukaya categories after Haiden-Katzarkov-Kontsevich.

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arXiv2021

Derived equivalence classification of Brauer graph algebras

We classify Brauer graph algebras up to derived equivalence by showing that the set of derived invariants introduced by Antipov is complete. These algebras first appeared in representation theory of finite groups and can be defined for any suitably decorated graph on an oriented surface. Motivated by the connection between Brauer graph algebras and gentle algebras we consider $A_{\infty}$-trivial extensions of partially wrapped Fukaya categories associated to surfaces with boundary. This construction naturally enlarges the class of Brauer graph algebras and provides a way to construct derived equivalences between Brauer graph algebras with the same derived invariants. As part of the proof we provide an interpretation of derived invariants of Brauer graph algebras as orbit invariants of line fields under the action of the mapping class group.

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arXiv2020

Spherical objects, transitivity and auto-equivalences of Kodaira cycles via gentle algebras

This paper studies the class of spherical objects over any Kodaira $n$-cycle of projective lines and provides a parametrization of their isomorphism classes in terms of closed curves on the $n$-punctured torus without self-intersections. Employing recent results on gentle algebras, we derive a topological model for the bounded derived category of any Kodaira cycle. The groups of triangle auto-equivalences of these categories are computed and are shown to act transitively on isomorphism classes of spherical objects. This answers a question by Polishchuk and extends earlier results by Burban-Kreussler and Lekili-Polishchuk. The description of auto-equivalences is further used to establish faithfulness of a mapping class group action defined by Sibilla. The final part describes the closed curves which correspond to vector bundles and simple vector bundles. This leads to an alternative proof of a result by Bodnarchuk-Drozd-Greuel which states that simple vector bundles on cycles of projective lines are uniquely determined by their multi-degree, rank and determinant. As a by-product we obtain a closed formula for the cyclic sequence of any simple vector bundle on $C_n$ as introduced by Burban-Drozd-Greuel.

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arXiv2019

On auto-equivalences and complete derived invariants of gentle algebras

We study triangulated categories which can be modeled by an oriented marked surface $\mathcal{S}$ and a line field $\eta$ on $\mathcal{S}$. This includes bounded derived categories of gentle algebras and -- conjecturally -- all partially wrapped Fukaya categories introduced by Haiden-Katzarkov-Kontsevich. We show that triangle equivalences between such categories induce diffeomorphisms of the associated surfaces preserving orientation, marked points and line fields up to homotopy. This shows that the pair $(\mathcal{S}, \eta)$ is a triangle invariant of such categories and prove that it is a complete derived invariant for gentle algebras of arbitrary global dimension. We deduce that the group of auto-equivalences of a gentle algebra is an extension of the stabilizer subgroup of $\eta$ in the mapping class group and a group, which we describe explicitely in case of triangular gentle algebras. We show further that diffeomorphisms associated to spherical twists are Dehn twists.

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arXiv2018

A geometric model for the derived category of gentle algebras

In this paper we construct a geometric model for the triangulated category generated by the simple modules of any graded gentle algebra. This leads to a geometric model of their perfect derived categories and by a recent paper of Booth, Goodbody and the first author also of their derived categories of objects with finite-dimensional cohomology. The construction is based on the ribbon graph associated to a gentle algebra in the work of the third author, and is linked to partially wrapped Fukaya categories by the work of Haiden, Katzarkov and Kontsevich and to derived categories of coherent sheaves on nodal stacky curves by the work of Lekili and Polishchuk. The ribbon graph gives rise to an oriented surface with boundary and marked points in the boundary. We show that the homotopy classes of curves connecting marked points and of closed curves are in bijection with the isomorphism classes of indecomposable objects in the derived category of the graded gentle algebra. Intersections of curves correspond to morphisms and resolving the crossings of curves gives rise to mapping cones. The Auslander-Reiten translate corresponds to rotating endpoints of curves along the boundary. Furthermore, we show that the surface encodes the derived invariant of Avella-Alaminos and Geiss.

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