My research aims at applications in algebraic geometry by enhancing the toolbox of symbolic computations with methods of constructive category theory.

CAP project

CAP (Categories, Algorithms, Programming) is a software project for constructive category theory written in GAP. It facilitates both the realization of specific instances of categories and the implementation of generic categorical algorithms. With my colleague Sebastian Gutsche I gave an introduction at the Université Paris Diderot to constructive category theory and its applications to homological algebra. CAP is the cornerstone of my current research for experimenting and computing.

Trying CAP in a Jupyter notebook

CAP tutorials

CAP Days (conference)

Sebastian Gutsche, Sebastian Posur, Øystein Skartsæterhagen, On the Syntax and Semantics of CAP, In: O. Hasan, M. Pfeiffer, G. D. Reis (eds.): Proceedings of the Workshop Computer Algebra in the Age of Types,Hagenberg, Austria, 17-Aug-2018, published at
Sebastian Gutsche, Sebastian Posur, CAP: categories, algorithms, programming, Computeralgebra-Rundbrief (2019), Ausgabe 64, 14-17.

Publications and preprints

Sebastian Posur, A constructive approach to Freyd categories.

Peter Freyd’s universal way of equipping an additive category with cokernels can be seen as an abstraction of the usual model of finitely presented modules in computer algebra. An implementation of the so-called Freyd categories in CAP will make it possible to compute even with finitely presented functors. I gave a talk about this idea at the first annual meeting of the DFG collaborative research center SFB-TRR 195. The slides are available here.

Sebastian Posur, Linear systems over localizations of rings, Arch. Math. (2018) 111: 23.

Computable rings are exactly those rings for which the category of finitely presented modules is computable abelian (via the Freyd category construction). In this paper I discuss the computability of localizations of rings.

Sebastian Posur, Constructing equivariant vector bundles via the BGG correspondence, Journal of Symbolic Computation 91 (2019), 57 – 73, MEGA 2017, Effective Methods in Algebraic Geometry, Nice (France), June 12-16, 2017.

In this paper I apply the computational framework for equivariant graded modules over the exterior algebra that I established in my PhD thesis to the problem of constructing vector bundles on projective space. The corresponding CAP packages are

  • GroupRepresentationsForCAP: an implementation of a skeletal version of the semisimple tensor category of finite dimensional group representations
  • InternalExteriorAlgebraForCAP: an implementation of the category of internal modules over an internalized version of the exterior algebra

Sebastian Posur, Atom spectra of graded rings and sheafification in toric geometry, Journal of Algebra (2019), Volume 534, 207 – 227.

The theory of atom spectra of abelian categories was introduced by Ryo Kanda. Applied to the category of finitely presented graded modules, this theory yields a geometric criterion for deciding which objects sheafify to zero in the context of toric geometry, and will hopefully lead to improvements of algorithmic approaches in the future.

Mohamed Barakat, Markus Lange-Hegermann and Sebastian Posur, Elimination via saturation.

One can eliminate using an arbitrary monomial order. In particular, no elimination order is needed.

Sebastian Posur, Methods of constructive category theory.

This paper serves as an introduction to constructive category theory. It is shown how the philosophy of category constructors makes it possible to compute sets of natural transformations between finitely presented functors. Moreover, it is demonstrated how to perform diagram chases in homological algebra in a constructive way.


Constructive Category Theory and Applications to Equivariant Sheaves, PhD thesis, Siegen University.
G-equivariant Coherent Sheaf Cohomology, Master thesis, RWTH-Aachen University.
Axiomatik für lineare Differentialgleichungssysteme mit konstanten Koeffizienten und Anwendungen auf Invariantentheorie, Bachelor thesis, RWTH-Aachen University.

Non-refereed publications

Sebastian Posur, The search for low rank vector bundles, MFO report no. 25 (2013).