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tirling s ø ren moved to industry after finishing his phd colin is now retired luca you receive one of the two concur tot awards for the period 1990 1993 for the paper bisimulation equivalence is decidable for all context free processes you published with s ø ren christensen and colin stirling at concur 1992 could you tell us briefly what the context for that work was and how it came about hans i was working on my phd which was on the topic of decidability of behavioural equivalences for process calculi that allow processes to have infinite state spaces baeten bergstra and klop had proved that strong bisimilarity is in fact decidable for the bpa calculus even though the class of trace languages for bpa is exactly the class of context free languages the result only held for normed bpa processes that is processes that could always terminate colin was my supervisor in edinburgh and he suggested that i try to find a simpler proof of that result much can be said about the original proof but simple it was not this turned out to be really interesting i met didier caucal from irisa who had found a nice much shorter proof that relied on finite characterizations and i was able to use his notion of self bisimilarity to prove that a tableau system for bisimilarity was sound and complete wrt bisimilary jan friso groote who was a phd student at the cwi at the time and myself proved that all other known equivalences are undecidable for normed bpa and therefore a fortiori also for full unnormed bpa but one problem that colin and i were never able to solve was if the decidability result also held for the full bpa calculus søren arrived in edinburgh in 1990 and we shared a flat there in st leonards bank just across from holyrood park we hung out with the same people in the lfcs many of whom were clever italians such as davide sangiorgi and clever quasi italians such as marcelo fiore it is no surprise that søren and i often discussed our work or that he given that colin was also his supervisor moved on to study decidability problems as well the only difference being that søren was mostly interested in the bpp calculus that has parallel composition instead of sequential composition when i left edinburgh at the end of august of 1991 colin and i had managed to make some progress on the decidability problem for unnormed bpa we realised that a finite characterization of the maximal bisimulation á la caucal was the way ahead but the actual characterization escaped us when i returned for my viva in december søren had found an important lemma on finite characterizations and everything fell into place the decidability result relied on proving that bisimilarity is semi decidable using the finite characterization non bisimilarity was already known to be semi decidable luca both s ø ren christensen and you wrote the award winning paper as phd students in edinburgh could you tell us about the research environment in edinburgh at that time and the influence it had on you then and in the rest of your career hans the lfcs in edinburgh was a great place to be at the time robin milner was still around and busy working on the pi calculus together with davide sangiorgi who was his student at the time watching them at the blackboard was interesting even though the rest of us often could not follow their discussions gordon plotkin was there and still is fortunately rod burstall was still active and so was mike fourman plus many many others there was always someone to talk to and it was perfectly legitimate to have an interest in basic research unlike the other professors colin had no background in maths he was originally a philosopher and maybe that was why he was trying to avoid heavy mathematical lingo trying to be simple and precise in his writing at the same time i learned a lot from him also in that respect after søren returned to denmark he left academia and got a job in the private sector he still lives near aarhus sadly we lost touch with each other in all likelihood because our lives turned out so different we got back in touch recently when we were told about the reward and we look forward to finally meeting each other again colin retired recently and i really hope to see him again when i am able to travel to scotland again some day luca how much of your later work has built on your award winning paper what follow up results of yours are you most proud of and why hans i worked on decidability issues for another few years after that but there was no one to talk to in aalborg or rather there was no one there that shared my interest in the area at the time what is more the open problems that remained were major challenges some were only solved much later such as the decidability of bisimilarity for pushdown automata a result due to sénizergues the proof later greatly simplified by colin or remain open such as the decidability of weak bisimilarity for bpa eventually i drifted down the slippery slope and became interested in other related topics and focused somewhat more directly on program properties the follow up result that most directly relates to my paper with søren and colin is the result from 1994 that bisimilarity is also the only equivalence that is decidable for bpp this is a result that i am also quite proud of it grew out of discussions with javier esparza and yoram hirshfeld when i was back in edinburgh for a while in 1993 ironically there was a subtle flaw in the proof that naoki kobayashi discovered many years later naoki and one of his student found out how to repair the proof and the three of us co authored the journal version that came out many years later the reason why naoki became interested in bpp was that he was trying to use the calculus as a behavioural type system for a pi calculus as it happened this was also the route that i had taken those of my later results that i am most proud of have to do with this area my work on type systems for psi calculus concur 2011 and later and a session type system for bounded name use in the pi calculus acta informatica 2019 the latter paper also has a concur connection it grew out of attending a talk by roland meyer at concur 2013 and wondering why they were not trying to characterize notions of name boundedness in the pi calculus by means of a type system it turned out that meyer et al were not familiar with type systems at all it took me an awful long time to work out a type based characterization using session types as you can probably tell as you can tell i have not worked on bisimulation for quite some time not that there is anything wrong with bisimulation of course but if one wants results that are applicable for automated reasoning about program behaviour decidability is important one can then either choose to go for a less expressive model of computation which is what researchers in the model checking community do or keep the model of computation and go for sound but incomplete characterizations of program properties which is what researchers interested in type systems and related static analysis methods do i ended up in the latter camp luca to your mind what are the most interesting or unexpected uses in the literature of the notions and techniques you developed in bisimulation equivalence is decidable for all context free processes hans bpa bpp and similar process calculi can be used as the basis of behavioural type systems and i am thrilled that my old research interests and my current research interests are so directly related in 2018 i discovered that vasco vasconcelos and peter thiemann had devised a session type system for a functional programming language in which session types were not regular expressions which is essentially what they are in the original work by honda kubo and vasconcelos but bpa processes vasco and peter knew that type equivalence should be decidable but they were not so familiar with our results from the early 1990s at popl 2019 i attended a talk by andreia mordido one of vasco s collaborators and she mentioned our paper from concur 1992 later that day i ended up talking to andreia and vasco about my work on bpa from all those years ago luca the last forty years have seen a huge amount of work on process algebra and process calculi however the emphasis on that field of research within concurrency theory seems to have diminished over the last few years even in europe what advice would you give to a young researcher interested in working on process calculi today hans that they should still work on process calculi there remains a lot of interesting work to be done one reason why research topics drift in and out of focus is simply that researchers lose interest it is hardly ever the case that a topic runs out of interesting research questions another reason is rather grim basic research is nowhere near as well respected as it used to be if you look at the funding schemes that we have today only the very successful few can get funding for basic research i am not among them and it does get frustrating quite often most topics these days deal with applied research there is nothing wrong with applied research per se some of what i do is on that side of things but there has to be more to research than that luca what are the research topics that currently excite you the most hans i have slowly become more and more interested in programming language theory and some of my current collaborators have taken a similar route beginning in concurrency theory drifting into the world of behavioural type systems and finally wanting to apply all of their skills to actual programming languages right now mario bravetti adrian francalanza antónio ravara and myself are involved in working on a behavioural type system related to the mungo tool for a subset of java i am fortunately enough to have had three exceptionally clever and productive msc students involved as well and this has been extremely helpful mungo is in many ways the work of simon gay and ornela dardha at glasgow university and they too began their careers working on process calculi luca you have a keen interest in teaching at university level is there anything you think we should do better in conveying the beauty and usefulness of theoretical computer science in general and concurrency theory in particular to our students today do you have any thoughts you d like to share with us on how to combine digital resources and in person meetings in the teaching of subjects in the theory of computing hans personally i think there is a tendency to present any academic topic and in particular topics in the mathematical sciences broadly speaking in such a way that the definitions and theorems appear as if they fell from the heavens fully formed as any researcher will know that is certainly not how they came about in this way we focus a lot on what imre lakatos and karl popper called the context of justification a well honed presentation of a theory may appear to be beautiful but in my opinion the actual beauty is the one that one experiences when one has finally understood the theory and understands why the definitions and theorem turned out the way they did that is one must understand the context of discovery i think problem based learning which is something that we talk about a lot at aalborg university and at roskilde university is the key here because it puts the focus on student centered active learning i have lectured a lot over the years but since 2013 i have drifted away from traditional lectures towards flipped teaching in which i use pre recorded podcasts of my own making that students can watch whenever they want i then use the plenary sessions for activities that focus on active learning i by far prefer having a dialogue with students that focuses on the problems that they encounter to the monologue style of a lecture all my teaching activities are now flipped and i am happy to say that some of my colleagues are now thinking along similar lines it is always good to have someone to talk to posted by luca aceto at 9 29 pm no comments email this blogthis share to x share to facebook share to pinterest wednesday june 17 2020 logic mentoring workshop 2020 karoliina lehtinen asked me to encourage young researchers of all ages to attend this year s edition of the logic mentoring workshop see here for information the set of speakers is top class registration is free and i am sure that attending the event would be beneficial to many spread the news fwiw i gave a talk at the 2017 edition of the event and thoroughly enjoyed it even though the event is targeted at students from senior undergraduates to graduates i feel that i always learn something new from attending this kind of workshops talks posted by luca aceto at 4 10 pm no comments email this blogthis share to x share to facebook share to pinterest newer posts older posts home subscribe to posts atom pages home blog archive 2026 11 september 1 august 2 may 1 april 2 march 3 january 2 2025 9 june 1 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