Sunday, May 12, 2019

ICE-TCS Theory Day 2019

This post also appears on the ICE-TCS blog.

On Friday, 3 May, ICE-TCS hosted its 15th annual Theory Day. The event consisted of two 45-minute presentations by Ravi Boppana (Department of Mathematics, MIT) and Exequiel Rivas (Inria Paris - Rocquencourt, France), and three ten-minute presentations by ICE-TCS researchers highlighting some of the recent research directions pursued by members of the centre.

Ravi Boppana kicked off the Theory Day with a wonderfully paced talk on his work with Ron Holzman on Tomaszewski’s problem on randomly signed sums. The problem is as follows. Let , , ..., be real numbers whose squares add up to 1.  Consider the signed sums of the form .  Can there be more signed sums whose value is greater than 1 then those whose value  is at most 1? Holzman and Kleitman (1992) proved that at least 3/8 of these sums satisfy .  In his talk, Ravi showed us the main ideas Holzman and he used to improve the bound to 13/32.

Computational effects model the interaction of computer programs with their environment. In his talk, Exequiel Rivas taught us how monads can be used to capture computational effects (a research programme that started with Moggi's award-winning work), and then, discussed some attempts to incorporate merging operations in the monadic picture.

Two of the short talks were given by Henning A. Úlfarsson and Elli Anastasiadi. Henning described the work of his group on  a tool, called the CombSpecSearcher, that automates the methods used by combinatorialists to prove some of their theorems, The tool is able to prove results featured in dozens of research papers. Watch this space for updates on its development and for its successes!

Elli Anastasiadi, a PhD student who is already playing an important role for the centre, gave a clear seven-minute introduction to fine-grained complexity and to the notion of fine-grained reduction.

The 2019 Theory Day was well attended, at least by the standards of a TCS event in Iceland. If all goes well, we'll be back next year.

Thursday, April 18, 2019

The Complexity of Identifying Characteristic Formulae

One of the classic results in concurrency theory is the Hennessy-Milner Theorem. This result states that
  1. two bisimilar states in a labelled transition system satisfy exactly the same formulae in a multi-modal logic now called Hennessy-Milner logic, and 
  2. two states in a labelled transition system that satisfy a mild finiteness constraint (called image finiteness)  and enjoy the same properties expressible in Hennessy-Milner logic are bisimilar.
See, for instance, Section 1.2 in these notes by Colin Stirling for an exposition of that result. A consequence of the Hennessy-Milner Theorem is that whenever two states p and q in a labelled transition system are not bisimilar, one can come up with a formula in Hennessy-Milner logic that p satisfies, but q does not. Moreover, for each state p in a finite, loop-free labelled transition systems, it is possible to construct a formula F(p) in Hennessy-Milner logic that completely characterizes p up to bisimilarity. This means that, for each state q, p is bisimilar to q if, and only if, q satisfies F(p). The formula F(p) is called a characteristic formula for p up to bisimilarity. One can obtain a similar result for states in finite labelled transition systems by extending Hennessy-Milner logic with greatest fixed points.


Characteristic formulae have a long history in concurrency theory. However, to be best of my knowledge, the complexity of determining whether a formula is characteristic had not been studied before Antonis Achilleos first addressed the problem in this conference paper. In that paper, Antonis focused on the complexity of the problem of determining whether a formula F is complete, in the sense that, for each formula G, it can derive either G or its negation.

Our recent preprint  The Complexity of Identifying Characteristic Formulae extends the results originally obtained by Antonis to a variety of modal logics, possibly including least and greatest fixed-point operators. In the paper, we show that completeness, characterization, and validity have the same complexity — with some exceptions for which there are, in general, no complete formulae. So, for most modal logics of interest, the problem is coNP-complete or PSPACE-complete, and becomes EXPTIME-complete for modal logics with fixed points. To prove our upper bounds, we present a nondeterministic procedure with an oracle for validity that combines tableaux and a test for bisimilarity, and determines whether a formula is complete.

I think that there is still a lot of work that can be done in studying this problem, with respect to a variety of other notions of equivalence considered in concurrency theory, so stay tuned for further updates. 

Saturday, April 13, 2019

The ICE-TCS blog

After about 14 years, ICE-TCS has finally decided to have its own blog. In the past, I have covered ICE-TCS related news here and I will continue to do so. Those posts will also appear in the centre's blog.

Have a look at the new blog if you are interested in the work we do and in the events at our little centre in Reykjavik, Iceland.

ICSE 2019 ACM SIGSOFT Distinguished Paper Award to GSSI students

I am very pleased to inform you that Emilio Cruciani and Roberto Verdecchia, two third-year PhD students in CS at the GSSI,  and their coauthors Breno Miranda and Antonia Bertolino (member of the Scientific Committee of the PhD programme in CS at the GSSI) will receive an ICSE 2019 ACM SIGSOFT Distinguished Paper Award for their paper "Scalable Approaches for Test Suite Reduction". 
 
Distinguished Papers represent the very best contributions to the ICSE Technical Track, and are awarded to up to 10% of the papers. (ICSE is the premiere annual conference in the field of software engineering and is very competitive.) This is a remarkable achievement that reflects well on the authors, on CS@GSSI and on the institute as a whole. 

Congratulations to Antonella, Breno, Emilio and Roberto, not to mention the GSSI as a whole!

Thursday, April 11, 2019

EATCS Award 2019 to Thomas Henzinger

The EATCS has just announced that Thomas Henzinger is the EATCS Award 2019 recipient for "fundamental contributions to the theory and practice of formal verification and synthesis of reactive, real-time, and hybrid computer systems, and to the application of formal methods to biological systems." Congratulations to  the award committee---consisting of Artur Czumaj, Marta Kwiatkowska and Christos Papadimitriou (chair)---for their great choice, to Tom for a very well deserved award and to the TCS community at large.

Of course, Tom Henzinger needs no introduction. However, let me use this post to provide a bird's eye view of his career and of some of his many contributions to TCS, which would be enough for a good number of very successful research careers. The text below is largely due to Jean-Francois Raskin.


Biographical sketch Thomas A. Henzinger is the President of IST Austria (Institute of Science and Technology Austria), which, under his leadership, has quickly become one of the most impactful research institutes in the world. Before joining IST as its first president, Tom was Assistant Professor of Computer Science at Cornell University (1992-95), Assistant Professor (1996-97), Associate Professor (1997-98) and Professor (1998-2004) of Electrical Engineering and Computer Sciences at the University of California, Berkeley. He was also the Director of the Max-Planck Institute for Computer Science in Saarbruecken, Germany (1999) and Professor of Computer and Communication Sciences at EPFL in Lausanne, Switzerland (2004-09).

Tom is an ISI highly cited researcher and his h-index is 103, according to Google Scholar. He is a member of Academia Europaea, a member of the German Academy of Sciences (Leopoldina), a member of the Austrian Academy of Sciences, a Fellow of the AAAS, a Fellow of the EATCS, a Fellow of the ACM, and a Fellow of the IEEE. He was the recipient of the Milner Award of the Royal Society in 2015, of the Wittgenstein Award of the Austrian Science Fund and was granted an ERC Advanced Investigator Grant in 2010. He received the SIGPLAN POPL Most Influential Paper Award (2014), Logic in Computer Science Test-of-Time Award (2012), ACM SIGSOFT Impact Paper Award (2011), and Best Paper awards at SIGSOFT FSE and CONCUR.

Main scientific achievements Tom's research focuses on modern systems theory, especially models, algorithms, and tools for the design and verification of reliable software, hardware and embedded systems. His HyTech tool was the first model checker for mixed discrete-continuous systems.

Tom has made a large number of fundamental contributions to theoretical computer science. Here I will limit myself to mentioning a small number of research areas where he has been particularly prolific and influential.
  • The theory of timed and hybrid systems. Tom has defined and studied the expressiveness and the algorithmic complexity of several real-time extensions of temporal logics. He is one of the most important contributors to the theory of hybrid automata and to algorithms for the analysis of suchmodels. His papers on the subject are among the most cited ones in the field of computer-aided verification. As an example, his paper “Symbolic model checking for real-time systems” received a LICS Test-of-Time Award in 2012. He has also studied the gap that exists between mathematical models of systems (e.g. hybrid automata) and their implementation on physical hardware. To bridge this gap, he developed models of physical platforms such as Giotto and E-machines, and he devised ways to relate their semantics with the one of the abstract mathematical models.
  • Games for verification and control. Tom has introduced the logic ATL*, which extends LTL and CTL* with the ability to reason about strategies that can be played by coalitions of agents/components in models of multi-agent/component systems. He has contributed to the understanding of the potential of adopting concepts from game theory for modeling and reasoning about open systems. He has contributed to a large number of algorithmic advances for solving game-graph problems and to better understand their computational complexity.
  • From Boolean models to quantitative models for verification and synthesis. Tom has recently investigated how to shift from Boolean models to quantitative models. This research proposes quantitative generalizations of the paradigms that had success in reactive modeling, such as compositionality, property-preserving abstraction, model checking and synthesis. With those models, we can specify and reason about quantitative aspects, such as resource consumption, or compare the performance of different design solutions in embedded systems. He has obtained a substantial funding from the European Research Council to proceed along this promising line of research.
  • Foundations of software model checking. Tom has contributed substantially to the algorithms underlying the analysis of software systems by introducing the concept of lazy abstraction. Those ideas have been implemented in the highly influential tool Blast. This line of work was honoured with the Most Influential 2004 POPL Paper Award which he received in 2014.
  • Computational modelling of biological systems. Tom and his coworkers have shown that computational models are well suited to model the dynamics of biological systems. This is part of a broader research program that has the objective to show that concepts introduced to formalize reactive systems are helpful to model and reason about biological systems.
Those important theoretical contributions have always been developed with relevant practical applications in mind. Consequently, Thomas Henzinger has not only worked on the foundations of our subject, but he also transferred his theoretical ideas into practice by developing tools and by suggesting methodologies that could be used in applying his theoretical results.

Tom is a research leader who has had a profound influence on his PhD students and post-docs. To wit, several of his former students are now well-recognized researchers that enrich the life of our research community and now lead successful research groups.

Addendum 12 April 2019: In case you are interested, you can find a short interview I had with Tom Henzinger a while back here.

Thursday, April 04, 2019

Guarded recursion and unique fixed points: A pill of wisdom from a recent ICE-TCS seminar

Last Tuesday, Sergey Goncharov (FAU Erlangen-Nürnberg, Germany) delivered an ICE-TCS seminar entitled Guarded traced categories for recursion and iteration. In his talk, Sergey presented recent results, based on  joint work (in progress) with Lutz Schröder and Paul Blain Levy, on axiomatizing the theory of guarded fixed points with applications to process algebra in monad-based form, where the notion of guardedness originated. 

In the classic theory of process calculi without internal actions, an equation

X = P(X) 

is guarded if each occurrence of the variable X in the expression P(X) occurs within the scope of some prefixing operator. A classic result in the theory of, for instance, Milner's Calculus of Communicating Systems is that guarded equations have unique solutions modulo bisimilarity. For instance, the equation

X = a.X 

denotes the process rec X. a.X that executes action a indefinitely. If one interprets the above equation over the complete lattice of all languages of finite words over some alphabet, then its only solution is the empty language.

I freely admit that I was under the impression that guarded equations always had unique solutions, but I was wrong. There was a point in the talk at which Sergey showed that the above equation has more than one solution over other domains. Consider, for instance, the set of languages of infinite words over the singleton alphabet {a}. There are only two such languages, namely the empty language and {aω}, and both are solutions to the above equation. (This is not surprising, since a-prefixing is just the identity function over this family of languages.) The same holds true when we interpret that equation over the set of all languages of finite and infinite words over some alphabet.

This is probably not news to many, but I have learnt a lesson from attending the talk myself: Guardedness does not imply uniqueness of solutions.

To the young researchers of all ages out there: Go to talks and keep an open mind. You will learn a few bits of information that you might need in your future research projects or you might clarify some misconceptions you might have had, like I did last Tuesday.

Thursday, March 14, 2019

Seven PhD positions in CS at the GSSI

This call for PhD positions might be of interest to some of your students or you. Please distribute it as you see fit. 

The GSSI - Gran Sasso Science Institute offers seven PhD fellowships in Computer Science for the academic year 2019/20. One of those positions is earmarked for the development of efficient techniques and tools for the analysis of large genomic data and is supported by the genomic-research start-up Dante Labs srl. The official language for all PhD courses is English.

The fellowships are awarded for 4 years and their yearly amount is € 16.159,91 gross. All PhD students have free accommodation at the GSSI facilities and use of the canteen.

The application must be submitted through the online form available here by 18 June 2019 at 6 pm (Italian time zone). 

 The GSSI-Gran Sasso Science Institute is an international PhD school and a center for research and higher education in the areas of Physics, Mathematics, Computer Science and Social Sciences. Founded in 2012 in L’Aquila (Italy) as Center for Advanced Studies of the National Institute for Nuclear Physics (INFN) and then established in March 2016 as a School of Advanced Studies providing post-graduate education. Through a day-to-day collaboration and interaction, researchers and students have the opportunity to build a sound knowledge of the research methods and to experiment contamination of interests, innovative approaches and multicultural exchanges in all the GSSI activities. In addressing the complexity of today’s world, GSSI is committed to removing all barriers between its areas of study and research. The dissemination of scientific results towards society and the promotion of cultural events for generic public, citizens and schools are among GSSI goals.

See here for further details. 

Fully-funded four-year PhD scholarships at IMT Lucca




IMT School for Advanced Studies Lucca invites applications for PhD
positions in the Systems Science program, and specifically for its Computer
Science and Systems Engineering
<https://www.imtlucca.it/en/programma-dottorato/anno-accademico-2019-20/systems-science/csse>
track.

We carry out foundational, applied, and interdisciplinary research on the
modeling and analysis of systems, broadly construed. The SYSMA
<http://sysma.imtlucca.it/> research unit welcomes applications from
candidates in Computer Science with an interest in any of the following
topics:

- cloud computing;

- computational methods for the analysis of cyber-physical systems;

- cybersecurity;

- modeling and verification of concurrent, distributed, and self-adaptive
systems;

- program analysis;

- smart contracts and blockchain technology;

- software performance evaluation.

A non-exhaustive list of suggested PhD topics is available here
<https://sysma.imtlucca.it/phd/phd-projects/>. Prospective candidates are
warmly encouraged to get in touch with members
<https://sysma.imtlucca.it/people/> of the SYSMA unit for informal
enquiries.

The scholarship is for 4 years and consists of grant amounting to € 15,300
gross/year, in addition to free accommodation and board at the IMT Campus
<https://www.imtlucca.it/en/campus/overview>. PhD candidates have the
possibility to defend their thesis from the beginning of the fourth year of
the program, but no earlier as per Italian legislation.

The initial start date of the PhD program is 1 November 2019. The working
language at IMT is English.

Application deadline is 23 April 2019. Note that candidates who have not
obtained their undergraduate degree by the deadline can still apply, and
can be admitted if they graduate no later than 31 October 2019.

Applications must be submitted through the online form at:

https://www.imtlucca.it/en/programma-dottorato/ammissione/procedure

Tuesday, March 05, 2019

SIROCCO 2019 - Second Call for Papers

Michele Flammini, PC co-chair of this year's SIROCCO conference, asked me to post the second CFP for that event. Submit your paper and win a trip to the Abruzzo region this summer! (In case you need them, here are ten reasons why you should. In my biased opinion, there are many more.)

SIROCCO 2019 - Second Call for Papers
 
 
26th International Colloquium on Structural Information and Communication Complexity (SIROCCO 2019)
July 1-4, 2019, L’Aquila, Italy
 
OVERVIEW. SIROCCO is devoted to the study of the interplay between structural knowledge, communication, and computing in decentralized systems of multiple communicating entities. Special emphasis is given to innovative approaches leading to better understanding of the relationship between computing and communication. SIROCCO has a tradition of interesting and productive scientific meetings in a relaxed and pleasant atmosphere, attracting leading researchers in a variety of fields in which communication and knowledge play a significant role. This year, SIROCCO will be held in L’Aquila, a beautiful historical city in the mountain side, 100km away from Rome.
 
SCOPE. Original papers are solicited from all areas of study of local structural knowledge, global communication, and computational complexities. Among the typical areas are distributed computing, communication networks, game theory, parallel computing, social networks, mobile computing (including autonomous robots), peer to peer systems, communication complexity, fault tolerant graphs, and probability in networks. Keeping up with the tradition of SIROCCO, new areas are always welcome.
 
SUBMISSIONS. Papers are to be submitted electronically through EasyChair at the following link: https://easychair.org/conferences/?conf=sirocco2019.
 
Authors are invited to submit their work in one of the two acceptable formats: regular papers and brief announcements. First, abstracts should be registered and later the full papers are due. A regular submission must report on original research and must contain results that have not previously appeared, and have not been concurrently submitted to a journal or a conference with published proceedings. It must be no longer than 12 single-column pages on letter-size paper using at least 11-point font, including tables and references. Up to 4 additional pages of figures may be included with the submission. Additional details may be included in a clearly marked appendix, which will be read at the discretion of the program committee. Full Proofs omitted for lack of space should appear in an appendix.
A submission for a brief announcement must be no longer than 3 single-column pages on letter-size paper using at least 11-point font. The title of a submission for a brief announcement must begin with “Brief announcement:”. Such submissions may describe work in progress or work presented elsewhere. A submission that is not selected for regular presentation may be invited for a brief announcement.
 
PUBLICATION. Regular papers and brief announcements will be included in the conference proceedings that will be published by Springer. Regular papers receive up to 15 pages, brief announcements receive up to 2 LNCS-style pages.
 
BEST STUDENT PAPER AWARD. A prize will be given to the best student regular paper. A paper is eligible if at least one author is a full-time student at the time of submission, and the contribution of the student is significant. This must be noted on the cover page. The program committee may decline to give the award or may split it.
 
INVITED SPEAKERS
Susanne Albers, TU München, Germany
Pierre Fraigniaud, CNRS and University Paris Diderot, France
Stefano Leonardi, Sapienza University of Rome, Italy
Merav Parter, Weizmann Institute of Science, Israel
 
SIROCCO 2019 will also feature a talk by Paola Flocchini, University of Ottawa, Canada – Recipient of the 2019 Prize for Innovation in Distributed Computing
 
KEY DATES (all in 2019)
Abstract registration: April 1
Full paper submission: April 7
SIROCCO notification: May 17
Proceedings version: TBD
Early registration: June 1
 
PROGRAM COMMITTEE
Dan Alistarh, IST Austria, Austria
James Aspnes, Yale University, USA
Alkida Balliu, Aalto University, Finland
Vittorio Bilò, University of Salento, Italy
Lelia Blin, Sorbonne University, France
Ioannis Caragiannis, University of Patras, Greece
Keren Censor-Hillel, Technion, Israel (co-chair)
Michele Flammini, GSSI & University of L’Aquila, Italy (co-chair)
Luisa Gargano, University of Salerno, Italy
Chryssis Georgiou, University of Cyprus, Cyprus
Magnús Halldórsson, Reykjavik University, Iceland
Tomasz Jurdzinski, University of Wroclaw, Poland
Fabian Kuhn, University of Freiburg, Germany
Toshimitsu Masuzawa, Osaka University, Japan
Alessia Milani, University of Bordeaux, France
Ivan Rapaport, Universidad de Chile, Chile
Dror Rawitz, Bar-Ilan University, Israel
Mordechai Shalom, Tel-Hai College, Israel
Jennifer Welch, Texas A&M University, USA
Prudence W.H. Wong, University of Liverpool, UK
Yukiko Yamauchi, Kyushu University, Japan
        
ORGANIZING COMMITTEE
Guido Proietti, University of L'Aquila, Italy (chair)
Gianlorenzo D’Angelo, GSSI, Italy
Mattia D’Emidio, University of L'Aquila, Italy
Michele Flammini, GSSI & University of L'Aquila, Italy
Luciano Gualà, University of Rome Tor Vergata, Italy
Ludovico Iovino, GSSI, Italy
Cosimo Vinci, University of L'Aquila, Italy
 
STEERING COMMITTEE
Shantanu Das, Aix-Marseille University, France
Rastislav Kralovic, Comenius University, Slovakia
Zvi Lotker, Ben Gurion University, Israel
Boaz Patt-Shamir, Tel Aviv University, Israel
Andrzej Pelc, University of Québec, Canada
Nicola Santoro, Carleton University, Canada
Sebastien Tixeuil, University Paris 6, France
Jukka Suomela, Aalto University, Finland