Weekly research seminar

2023-05-31, 14:15, 5050

Szymon Toruńczyk (MIM UW)

I will define a new graph parameter called flip-width. Graph classes of bounded flip-width include classes of bounded expansion of Nesetril and Ossona de Mendez, as well as classes of bounded twin-width of Bonnet, Kim, Thomasse, and Watrigant....

2023-05-24, 14:15, 5050

Mikołaj Bojańczyk (MIM UW)

**A categorical characterisation of linear regular functions**We consider linear regular string-to-string functions, i.e. functions that are recognized by copyless streaming string transducers, or any of their equivalent models, such as deterministic two-way automata. Although this class of functions is clearly important, in particular because of the many equi...

2023-05-17, 14:15, 5050

Liat Peterfreund (CNRS, LIGM, Paris-Est University)

**Querying incomplete numerical data: between certain and possible answers**Queries with aggregation and arithmetic operations, as well as incomplete data, are common in real-world databases, but we lack a good understanding of how they should interact. On the one hand, systems based on SQL provide ad-hoc rules for numerical nulls, on the other, theoretical research largely...

2023-05-10, 14:15, 5050

Nikolas Mählmann (University of Bremen)

**Monadically Stable Classes of Graphs**Intuitively, a class of graphs is monadically stable if first-order logic cannot encode arbitrary long linear orders in it. While originating from model theory, monadic stability generalizes many combinatorial properties of sparse graph classes such as planarity, bounded treewidth, bounded degree, ...

2023-04-26, 14:15, 5050

Yoàv Montacute (University of Cambridge)

The study of the relationship between logic and topology has a rich and extensive history. One way of exploring this relationship involves examining the formal languages of indiscrete spaces and their discrete representations. The talk will provide an introduction to this topic and its extension to ...

2023-04-19, 14:15, 5050

Lorenzo Clemente (MIM UW)

**Equality and zeroness problems of power series for combinatorial enumeration**A power series is constructible differentially finite (CDF) if it satisfies a system of polynomial differential equations [Bergeron & Reutenauer 1990, Bergeron & Sattler 1995]. CDF series generalise algebraic power series and find a natural application as exponential generating functions in combina...

2023-04-12, 14:15, 5050

Mikołaj Bojańczyk (MIM UW)

**On the growth rate of polyregular functions**Polyregular functions are string-to-string functions that have polynomial size outputs. I will show that a polyregular function has growth rate O(n^k) if and only if it can be defined by an mso transduction of dimension k. On the other hand, the former two conditions are not equivalent to k-pebble a...

2023-04-05, 14:15, 5050

Roland Guttenberg (TU Munich)

**Geometry of Vector Addition Systems**We will prove the missing line conjecture for VAS. The proof proceeds in three steps, all based on the basic units of VAS: Periodic sets. 1) Prove an easy special case. 2) Explain all closure properties of classes interesting for VAS. 3) The closure properties non-trivially combine to produce a...

2023-03-29, 14:15, 5050

Giannos Stamoulis (LIRMM, Université de Montpellier, CNRS)

**Model Checking Disjoint-Paths Logic on Topological-Minor-Free Graph Classes**Disjoint-paths logic, denoted FO+DP, extends first-order logic (FO) with atomic predicates dp_k[(x_1, y_1),…,(x_k, y_k)], expressing the existence of internally vertex-disjoint paths between x_i and y_i, for 1 ≤ i ≤ k. In this talk, we first show how to prove (fixed-parameter) tractability of ...

2023-03-22, 14:15, 5050

Wojciech Przybyszewski (MIM UW)

**Reproving combinatorial properties of nowhere-dense graphs using model theory**A pinnacle of sparsity theory, initiated by Ossona de Mendez and Nešetřil, is the result of Grohe, Kreutzer, and Siebertz which identifies subgraf-closed classes of graphs with FPT model checking as exactly nowhere dense classes. One of the key steps in the proof is characterizing nowhere dense cl...