| Links' Seminars and Public Events |   |
| 2022 | |
|---|---|
Fri 18th Nov 11:00 am 11:30 am | Seminar by Sarah Winter CANCELLED for COVID: we will attempt to reschedule this seminar to early 2023 Speaker: Sarah Winter — sarahwinter.net/ Title: A Regular and Complete Notion of Delay for Streaming String Transducers Abstract: The notion of delay between finite transducers is a core element of numerous fundamental results of transducer theory. The goal of this work is to provide a similar notion for more complex abstract machines: we introduce a new notion of delay tailored to measure the similarity between streaming string transducers (SST). We show that our notion is regular: we design a finite automaton that can check whether the delay between any two SSTs executions is smaller than some given bound. As a consequence, our notion enjoys good decidability properties: in particular, while equivalence between non-deterministic SSTs is undecidable, we show that equivalence up to fixed delay is decidable. Moreover, we show that our notion has good completeness properties: we prove that two SSTs are equivalent if and only if they are equivalent up to some (computable) bounded delay. Together with the regularity of our delay notion, it provides an alternative proof that SSTs equivalence is decidable. Finally, the definition of our delay notion is machine-independent, as it only depends on the origin semantics of SSTs. As a corollary, the completeness result also holds for equivalent machine models such as deterministic two-way transducers, or MSO transducers. This is joint work with Emmanuel Filiot, Ismaël Jecker, and Christof Löding. |
Fri 21st Oct 11:00 am 12:00 pm | Online seminar by Pierre Pradic Speaker: Pierre Pradic — perso.ens-lyon.fr/pierre.pradic/ Title: Synthesizing Nested Relational Queries from Implicit Specifications Abstract: Derived datasets can be defined implicitly or explicitly. An implicit definition (of dataset O in terms of datasets I⃗ ) is a logical specification involving the source data I⃗ and the interface data O. It is a valid definition of O in terms of I⃗ , if any two models of the specification agreeing on I⃗ agree on O. In contrast, an explicit definition is a query that produces O from I⃗ . Variants of Beth's theorem state that one can convert implicit definitions to explicit ones. Further, this conversion can be done effectively given a proof witnessing implicit definability in a suitable proof system. In this talk, I will discuss an analogous effective implicit-to-explicit result for nested relations: implicit definitions, given in the natural logic for nested relations, can be effectively converted to explicit definitions in the nested relational calculus NRC. I will first spend some time explaining what NRC is and what logic we use to describe implicit definitions of nested queries. Then I will present the results obtained in our papers, attempt to give some intuitions on the proof of the main theorem and say a few words on in particular the proof-theoretic techniques and concerns that come up (namely, cut-elimination and focussing) and how this can impact the complexity of extracting explicit definitions from proofs of implicit definability. Then if time allows I will discuss a more general model-theoretic result that we first used to give a non-constructive proof of our theorem, and some ideas that we have towards making it constructive and bounding the complexity of extracting explicit definitions. This is Joint work with Michael Benedikt and Christoph Wenhard. The main results I will be discussing were written up in arxiv.org/abs/2005.06503 and arxiv.org/abs/2209.08299.  Online |
