- Prof. Laurent Bartholdi
- Mathematics and Computer Science
- Campus E2 4, office 4.25
- Saarland University
- 66123 Saarbrücken 724W+J5M
- Phone: +49 681 302-3227
- Email: laurent.bartholdi@gmail.com
- 0000-0002-1243-6384
- Secretary: Christine Wilk-Pitz, office 4.26
- Secr. phone: +49 681 302-3430
- Secr. email: charly@math.uni-sb.de

## Members

- Laurent Bartholdi
- Leon Pernak
- Emmanuel Rauzy
- Christine Wilk-Pitz

## Research

Publications in Google Scholar and arXiv.Curriculum Vitae in PDF.

I am currently interested in interactions between theoretical computer science and, on the one hand, self-similar behaviour in group theory and algebra, and on the other hand its applications to complex and symbolic dynamical systems.

Self-similar, or *fractal*, objects abound in mathematics; depending on context, they mean a space containing several almost disjoint copies of itself as subspaces; a group containing the direct product of copies of itself as a subgroup; or an algebra containing a matrix algebra over itself as a subalgebra. The *fractalness* is algebraically encoded via the collection of inclusion maps of these subobjects in their common parent.

A self-similar group may be associated with any complex dynamical system, and yields an extremely potent algebraic invariant of that dynamical system up to isotopy and conjugation. I currently explore more deeply the connection between complex dynamics and fractal groups, and use it to extend the classification of degree-two polynomials (described by points in the Mandelbrot set) to arbitrary-degree rational functions.

All these questions immediately raise decidability and complexity questions: one the one hand, one would like an understanding of which problems are undecidable, usually in terms of their ability to simulate arbitrary Turing machines, and how high in the complexity hierarchy are the decidable problems; on the other hand, not only theoretical study, but also practical implementation of algorithms is fundamental in exploration and experimentation.

## Teaching, SoSe 2023

- Automata groups, Tue+Wed 10:15–12:00 in E2.4 SR6-->
- Oberseminar "algebra and number theory", Tue 16:15–17:00 in SR6
- [Ober]seminar Computability in mathematics, by Lean Pernak and Emmanuel Rauzy
- Seminar "groups and automata", TBD
- Seminar "computer-assisted proofs in LEAN", Wed 2:00–3:30 online

## Software

- The FR GAP package, for manipulation of groups and semigroups generated by automata
- The IMG GAP package, for manipulation of iterated monodromy groups and complex dynamics
- The Float GAP package, for manipulation of real, complex, interval etc. floating-point numbers
- The anq system, a C++ implementation of a quotient algorithm for groups, Lie algebras and associative algebras

## Other

Unicode cut&paste sheet.