ABOUT 
Computer Science Internal SeminarsThe Internal Seminar Series is a relaxed forum for members of the Department to present their current research and discuss ideas of interest. Invited speakers are also welcome, in particular for presentations that might be too specialised for a general computer science audience as on the Friday's seminar. Semester 2Room this semester is BEN LT3.Seminar programme
Seminar detailsSecure message transmison on directed graphs (Slides)
Ludovic Renou (Leicester, Department of Economics) Semester 1Room this semester is ENG LT1.Seminar programme
Seminar detailsParameterization of graph separation problems: current results and further challenges Igor Razgon () Fixedparameter computation is a methodology
of coping with NPhardness that, under certain conditions,
allows to solve hard optimization problems both precisely
and efficiency. According to this methodology, a problem
is associated with a parameter $k$ and solved by an
exponential time algorithm whose exponential part solely
depends on the parameter while dependnency on the input
size $n$ is uniformly polynomial, i.e. the degree of the
polynomial is a constant, usually not greater than 3. When
the parameter is small, we get a lowpolynomial algorithm
solving an NPhard problem to optimality. Problems that
can be solved in this way are called fixedparameter
tractable (FPT). The parameter choice depends on the
particular application and can be, for example, maximum or
minimum allowed size of the output or some structural
measure of the input, e.g. treewidth. This talk is
related to parameterization of graph separation problems
where the task is to find the smallest set of vertices
separating some predefined pairs of terminals, possibly
under additional constraints. In the first part of this
talk, I will informally define a fixedparameter algorithm
and describe two fixedparameter algorithms for the Vertex
Cover problem. In the second part of the talk I will
describe an algorithm for solving the multiway cut problem
and show how the main idea of this algorithm has helped to
show the fixedparameter tractability of such challenging
graph separation problems as Directed Feedback Vertex Set,
minimum 2CNF deletion, and Multicut. In the final part of
the talk I will outline the current challenges related to
the parameterization of graph separation problems. The
main challenge is kernelization of these problems, i.e.
designing a polynomialtime procedure that squeezes the
initial instance of the problem so that its size
polynomially depends on the parameter. Such procedure
being applied at the preprocessing stage can significantly
enhance any algorithm solving the given problem and hence
understanding kernelizability of parameterized problems is
a research direction of a great practical importance. The talk is selfcontained and assumes no prior
familiarity with the area.


Author: Alexander Kurz (kurz mcs le ac uk), T: 0116 252 5356. 