- Start: Thursday August 15th 2019 at 14:00
- Finish: Friday August 23th 2019 at noon

# Detailed Schedule

## School (August 15 - August 17)

The school is meant for beginners in computer algebra, to learn about the capabilities of the CAS GAP and Singular. It consists of six sessions, each three hours long, consisting of a lecture about the topic and a long hands-on session, in which participants can exercise their newly gained knowledge. The goal is that after this school, participants should be able to work with the developers in the following workshops. The sessions are:

## August 15, 14:00 - 17:00: Best practices software development (Max Horn)

In this course, you will learn about some best practices in (mathematical) software development: We will discuss the importance of, and how to do, source code version control and issue tracking. There will be practical demonstrations using the git version control system, and GitHub for collaborative work, and a set of exercises to train these skills.## August 15, 19:00 - 20:30, and August 16, 9:30 - 11:00: GAP for beginners (Michael Torpey)

This lesson gives an introduction to GAP. It is centered around a common task of searching in the Small Groups Library for interesting examples and counterexamples, and a particular research problem in which we will be interested is to- basic constructions of the GAP programming language,
- ways to find necessary information in the GAP system, and
- good design practices to organize GAP code into complex programs.

## August 16, 11:30 - 13:00, and 14:00 - 15:30: Advanced Topics in GAP (Thomas Breuer)

Motivated by a mathematical question, we will develop GAP functions for answering questions from various areas (combinatorics, group theory, representation theory). We will combine them with available functionality, create new kinds of objects, and extend GAP's capabilities in special situations.## August 16, 16:00 - 19:00: Singular for beginners (Christian Eder, Andreas Steenpaß, Isabel Stenger)

We will give an introduction to Singular starting from the very first line of code, and show how it can be used for theoretical research. On the practical side, the participants are encouraged to write their own Singular code, ranging from basic polynomial computations to more advanced projects such as writing a Singular library for their own research.## August 17, 9:30 - 12:30: Parallel modular algorithms in Singular (Christian Eder, Andreas Steenpaß, Isabel Stenger)

Modular algorithms are an important tool for tackling research problems in computational algebra whenever coefficient swell is an issue. At the same time, they offer a relatively easy way for parallelization. The basic idea is to make use of the Chinese remainder theorem for decomposing the original computation over some given ring into several computations over different rings for which the arithmetic is computationally easier, and to recombine the results. In the simplest case, a computation over the rational numbers is decomposed into computations over fields of order p for several primes p, but modular methods can also be applied to, for example, number fields and function fields.We will give an introduction to several applications of this principle as well as to the existing implementations of modular algorithms in Singular. We will also discuss both technical and theoretical problems which arise from this approach. For more complex applications such as Gröbner basis computations for example, we often face the problems how the modular results can be recombined and how correctness of the final result can be ensured.

## August 17, 14:00 - 17:00: CAP: Categories, algorithms, programming (Sebastian Posur)

The CAP Days school provides a gentle introduction to the basic notions of category theory and their realization in CAP (categories, algorithms, programming), a software project for constructive category theory written in GAP. We learn how to compute with finite dimensional vector spaces and finitely presented modules using the unifying language of abelian categories, and write generic algorithms that work in arbitrary abelian categories, e.g., for the intersection of subobjects. You may test and play with CAP's categorical language by running Jupyter notebooks interactively in Binder: click here to launch Binder.## Sunday Hike (August 18)

For Sunday, we will plan a hike together. Information will be given during the first days at Lambrecht.

## Mini conference (August 19)

Participants are welcome to give talks about problems they have tackled or want to tackle using computer algebra systems. Furthermore, developers of CAS who want to present their work are welcome to present it. Please note that each talk should contain either a research question to tackle with CAS, or a demo of CAS features.

## 10:00 - 10:30: Martin Bies: Monoidal structures in Freyd categories

For a given additive category C, one can construct a new category which is known as its Freyd category. In this category, a morphism of C is interpreted as an object. The morphisms in the Freyd category are understood as commuting squares of morphisms in C up to a certain equivalence relation. The package "FreydCategories", which is part of the CAP_project, provides an implementation of this functionality. For example, this package can be used to model the category of f.p. graded modules. Further upshots include toric sheaves and their cohomologies. The application to sheaf cohomologies and toric sheaves points out the need for a monoidal structure and internal Hom in Freyd categories. Therefore, we have recently focused on implementing mechanisms which derive these structures on Freyd categories from corresponding structures of the underlying additive category. I will elaborate on these developments.## 11:00 - 11:30: Wilf Wilson: Searching in permutation groups with directed graphs

The current state-of-the-art approach for problems like set stabilisers, intersections, and normalisers in permutation groups is called partition backtrack. In essence, partition backtrack performs a search that estimates the solution as the stabiliser of an ordered partition. With some collaborators, I am exploring how to take advantage of modern computational tools to replace ordered partitions in such searches by directed graphs. The idea is that a graph can be used to represent a group more precisely than an ordered partition, and therefore lead to smaller search spaces. I will talk about the mathematical progress that we have made, and the computational tools that have helped us along the way.## 11:30 - 12:00: Mahsa Sayyary Namin: The algebraic degree of the Fermat-Weber point

The Fermat-Weber point p* is the unique point that minimizes the sum of distances from n given points in the real Euclidean space. Given n points in general position in the real plane with non-zero integer coordinates, we determine the algebraic degree of p* over the field of rationals Q, i.e. we find the degree of the minimal polynomials of the coordinates of p* over Q.## 14:00 - 14:30: Yue Ren: Computing zero-dimension tropical varieties using modular techniques

In this talk, we will give a brief introduction to the concept of tropical varieties. We will discuss the applications and the challenges for the computation of zero-dimensional tropical varieties, and present a new approach using parallelization and modularization. This is joint work with Paul Goerlach (MPI MiS) and Leon Zhang (UC Berkeley)## 14:30 - 15:00: Johannes Flake: Computing the Monoidal Center of Deligne's Interporation Category Rep(S_t)

Deligne's interpolation categories are interesting examples for monoidal categories which can be described nicely using combinatorics and linear algebra. I will explain how objects in their monoidal centers can be constructed, and how I hope to produce more such objects or show that there are none using computer algebra.## 16:00 - 16:30: Manuel Delgado: Exploring N numerical semigroups in n milliseconds, with GAP

TBA## 16:30 - 17:00: Dominik Bernhard: Automorphism groups of simple graphs with few vertex-orbits

In this talk we will investigate how to construct automorphism groups of graphs with few vertex orbits. The base case is to construct automorphism groups with 2 vertex orbits. We will see how to describe their group theoretic structure and give hints towards an algorithm to construct these groups and how to generalize to more orbits.## Workshops (August 20 - 23)

From Tuesday to Friday participants are encouraged to work in several workshops on specific problems they are interested in or facing in computer algebra. If you want to definitely participate in a certain workshop, please mention it in the registration.

Each day, the workshops start at 9:30. Each organizer may set up their own schedule and mode of work.

Click here for the workshop HackMD

The workshops (and their specific organizers) are: