Informal PhD Student Seminars in Theoretical Computer Science

The PhD student seminars are intended to be informal and relaxed without the "polish" of the official seminars. The seminars can discuss for example recent results (your own or from an article) or survey some interesting topic. The intended audience is primarily PhD students, but everyone is welcome. Contact Björn Terelius if you want to give a seminar or receive email announcements about upcoming seminars.

More information about practical details are available in Swedish. There is also a page with previous seminars.

See also the page with the TCS groups official seminars.

Informal Seminars Fall 2011

• 09 Nov 2011 at 13:15 in room 1537
  ProMoVer: Modular Verification of Temporal Safety Properties
  (Siavash Soleimanifard, KTH CSC)

  In this talk, I will introduce ProMoVer, a tool for fully automated procedure-modular verification of Java programs equipped with method-local and global assertions that specify safety properties of sequences of method invocations. Modularity at the procedure-level is a natural instantiation of the modular verification paradigm, where correctness of global properties is relativized on the local properties of the methods rather than on their implementations, and is based here on the construction of maximal models for a program model that abstracts away from program data. This approach allows global properties to be verified in the presence of code evolution, multiple method implementations (as arising from software product lines), or even unknown method implementations (as in mobile code for open platforms). ProMoVer automates a typical verification scenario for a previously developed tool set for compositional verification of control flow safety properties, and provides appropriate pre- and post-processing. Modularity is exploited by a mechanism for proof reuse that detects and minimizes the verification tasks resulting from changes in the code and the specifications. The verification task is relatively light-weight due to support for abstraction from private methods and automatic extraction of candidate specifications from method implementations. I evaluate the tool on a number of applications from the smart card domain.

  This is a rehearsal talk for SEFM'11.

• 11 Oct 2011 at 10:15 in room 4523
  Approximating Graphic TSP by Matchings
  (Tobias Moemke, KTH CSC)

  We present a framework for approximating the metric TSP based on a novel use of matchings. Traditionally, matchings have been used to add edges in order to make a given graph Eulerian, whereas our approach also allows for the removal of certain edges leading to a decreased cost.

  For the TSP on graphic metrics (graph-TSP), the approach yields a 1.461-approximation algorithm with respect to the Held-Karp lower bound. For graph-TSP restricted to a class of graphs that contains degree three bounded and claw-free graphs, we show that the integrality gap of the Held-Karp relaxation matches the conjectured ratio 4/3. The framework allows for generalizations in a natural way and also leads to a 1.586-approximation algorithm for the traveling salesman path problem on graphic metrics where the start and end vertices are prespecified.

  This is a rehearsal talk for FOCS.

• 03 Oct 2011 at 13:15 in room 1439
  Provably Correct Control-Flow Graphs from Java Programs with Exceptions
  (Pedro de Carvalho Gomes, KTH CSC)

  We present an algorithm to extract flow graphs from Java bytecode, focusing on exceptional control flows. We prove its correctness, meaning that the behaviour of the extracted control-flow graph is an over-approximation of the behaviour of the original program. Thus any safety property that holds for the extracted control-flow graph also holds for the original program. This makes control-flow graphs suitable for performing different static analyses.

  For precision and efficiency, the extraction is performed in two phases. In the first phase the program is transformed into a BIR program, where BIR is a stack-less intermediate representation of Java bytecode; in the second phase the control-flow graph is extracted from the BIR representation. To prove the correctness of the two-phase extraction, we also define a direct extraction algorithm, whose correctness can be proven immediately. Then we show that the behaviour of the control-flow graph extracted via the intermediate representation is an over-approximation of the behaviour of the directly extracted graphs, and thus of the original program.

  This is a rehearsal talk for Foveoos 2011 (http://foveoos2011.cost-ic0701.org/) and feedback is very welcome.

• 27 Sep 2011 at 13:15 in room 1537
  Boolean polynomials and Gröbner bases: An Algebraic Approach to solving the SAT-problem
  (John Sass, SU)

  In this seminar we will discuss a method for solving the SAT-problem based on abstract algebra. The first step of the method is to convert the boolean formula to a set of \mathbb{Z}_2-polynomials, and looking at the ideal generated by these polynomials. If this ideal coincides with the entire polynomial ring, then and only then is the original boolean formula unsatisfiable, and we determine if this is the case by looking at a Gröbner basis for this ideal. Such a basis can be found using the Buchberger algorithm, which in the general case is a double exponential time algorithm.

  The seminar will be split into two 45-minute segments, with a ten minute break in between. In the first half of the seminar we will discuss the general method, including an introduction to multivariate polynomial reduction, Gröbner bases and the Buchberger algorithm. In the second half we will discuss the technical details, as well as implementation tricks and heuristic suggestions for the Buchberger algorithm which turn it into an exponential time algorithm. If there is time, we will also discuss how the method can be generalised and developed further.

• 29 Aug 2011 at 13:15 in room 1537
  Canonizable Partial Order Generators
  (Mateus de Oliveira Oliveira, KTH CSC)

  In a previous work we introduced slice graphs as a way to specify both infinite languages of directed acyclic graphs (DAGs) and infinite languages of partial orders. Therein we focused on the study of Hasse diagram generators, i.e., slice graphs that generate only transitive reduced DAGs, and showed that they could be used to solve several problems related to the partial order behavior of p/t-nets. In the present work we show that both slice graphs and Hasse diagram generators are worth studying on their own. First, we prove that any slice graph SG can be effectively transformed into a Hasse diagram generator HG representing the same set of partial orders. Thus from an algorithmic standpoint we introduce a method of transitive reducing infinite families of DAGs specified by slice graphs. Second, we identify the class of saturated slice graphs. By using our transitive reduction algorithm, we prove that the class of partial order languages representable by saturated slice graphs is closed under union, intersection and even under a suitable notion of complementation (cut-width complementation). Furthermore partial order languages belonging to this class can be tested for inclusion and admit canonical representatives in terms of Hasse diagram generators. As an application of our results, we give stronger forms of some results in our previous work, and establish some unknown connections between the partial order behavior of $p/t$-nets and other well known formalisms for the specification of infinite families of partial orders, such as Mazurkiewicz trace languages and message sequence chart (MSC) languages.