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PhD Seminar : Winter Term 2008/09Seminar programme
Seminar detailsFrom Reactions to Observations: the Directed Bigraphical Model
Davide Grohmann
(University of Udine, Italy) Linear Programming Models for Traffic Engineering Under Combined ISIS and MPLSTE Protocols
Anna Mereu
(University of Cagliari, Italy)
[ Slides ]
In the last few years, the Internet has exponentially expanded to a worldwide network connecting several millions of users. Such an explosion deeply impacts on the available bandwidth of IP backbone networks. Network Engineers have been building very large scale networks at their best, trying to understand beforehand (planning) and react just in time to the network events. Recently, new emerging Traffic Engineering (TE) techniques enable Internet Service Providers (ISP) to route the traffic along the network, offering the best service to their users in terms of throughput and latency, moving traffic from congested links to less loaded areas of the network. In particular, the MultiProtocol Label Switching (MPLSTE) networks enable ISPs to adopt qualityofservice (QoS) policies, setting up constrained label switched paths (LSPs). Furthermore, network survivability techniques have been developed to guarantee seamless communication services in case of network failures. Traffic management and restoration strategies are usually adopted in order to make a backbone network survivable, i.e., the network traffic load has to be distributed in such a way that a failure has the minimum (eventually, null) impact when it occurs; moreover, the traffic demands affected by the failure have to be suitably restored. The talk concerns the problem of minimizing the maximum link utilization of IP telecommunication networks under the joint use of the traditional ISIS/OPSF protocol and the MPLSTE technology. Both working conditions and single link failure scenarios are addressed. A Linear Programming mathematical model is proposed that, while optimizing the network utilization, provides optimal user performance, efficient use of network resources, and 100% survivability in case of single link failure. The hybrid approach takes advantage of both IGP and MPLSTE technologies providing a flexible tool for IP networks traffic engineers. The efficiency of the proposed appoach is validated by a wide experimentation performed on synthetic and real networks, both in working and failure conditions. The obtained results show that the maximum utilization of the network considerably reduces allowing a more efficient use of the network resources. Moreover, by setting a limited number of LSPs allows better results than those obtained by simply optimizing the IGP weights. However, using the hybrid approach, an optimized set of IGP weigths allows to further improve the network performance in case of failure. Coalgebras, the HennessyMilner property, and the Adjoint Functor Theorem
Raul Leal Rodriguez
(ILLC, University of Amsterdam)
[ Slides ]
It is well know that two states in a finite image Kripke frame are bisimilar iff they satisfy the same modal formulas, this is an instantiation of the HennessyMilner property. Coalgebras are a generalization of Kripke frames. In this talk we will use some elementary category theory to show how to generalize the HennessyMilner property to arbitrary coalgebras. Using this approach we will see that the HennessyMilner property is related to the existence of some solution set. Using this we will prove: The existence of a language with the HennessyMilner property for Tcoalgebras is equivalent to the existence of a final Tcoalgebra. Reasoning about effects: seeing the wood through the trees
Diana Fulger
(University of Nottingham)
Monads are widely used in the writing of computations with side effects, as a powerful tool allowing for more concise and intuitive expressions. One might expect reasoning about them to be just as straightforward, but wishful thinking didn't make it happen. We try to address the issue by means of a plain example: the labelling of a tree with distinct labels. We take a simple algorithm using a modifiable state, and prove it correct using elementary techniques in a first go, and monads / applicative functors the second time around. We thus obtain a factorisation of the labelling routine, the proof of which suggests taking some liberty with the type system, so as to describe some intriguing properties of the state applicative functor in interaction with function composition.
Unifying Theories of Programming, Categorically
Ondrej Rypacek
(University of Nottingham)
We deploy some category theory to compare two fundamental approaches to programming. Namely, algebraic data types and structural recursion play a central role in functional programming as they allow programmers to define recursive data structures and operations on them uniformly by induction. Likewise, in concurrent programming (such as idealised objectoriented programming), recursive hierarchies of components play a central role for the same reason. There is a semantical
correspondence between these two situations which we reveal and formalise categorically. In particular, we observe that (in some sense) welldefined programs with inductive data and coinductive behaviour are defined by a categorical distributive law of the structure of the data over the behaviour.
Dynamic Optimisation using Evolutionary Computation: An Introduction and Beyond
Dr. Philipp Rohlfshagen
(University of Birmingham, UK)
Evolutionary Algorithms (EAs) have been applied successfully
to a wide range of static optimisation problems. Many realworld
problems, however, possess numerous timevariant attributes that
require a continuous adaptation of the proposed solution. These
dynamic attributes pose many new challenges to the design and analysis
of novel EAs. In this talk, I will start with an introduction to
dynamic optimisation and will present some examples of dynamic
optimisation problems as well as existing approaches that have been
developed recently to address them. I will also present a formal
problem definition, placing special emphasis on the underlying problem
and the imposed dynamics. In the remainder of the talk I will focus on
the impact of timevariance in the combinatorial domain and will
present some results regarding the potential correlation of successive
environmental states in the subset sum problem.
MOEA/D and Its Hybridisation with Metaheuristics for Complex Multiobjective Optimisation Problems
Dr. Li Hui
(University of Nottingham, UK)
Over the past 20 years, the research on multiobjective evolutionary
optimisation has received a growing interest. In this talk, I will
introduce a recent new multiobjective approach, called
decompositionbased multiobjective evolutionary algorithm (MOEA/D). It
solves multiple single objective subproblems simultaneously by evolving
a population. The optima of these subproblems correspond to a set of
Paretooptimal solutions. Under the framework of MOEA/D, it is very
easy to incorporate other metaheuristics, such as simulated annealing,
differential evolution, memetic algorithm, greedy randomized adaptive
search, etc. Our experimental results show that MOEA/D is very promising
to solve complex multiobjective optimisation problems.
Categorical equational systems
ChungKil Hur
(University of Cambridge, UK)
[ Slides ]
We introduce two abstract notions of system of equations, called Equational System and Term Equational System. Equational Systems provide a very abstract notion of equation, which is general enough to represent nonclassical notions of equations as needed in modern applications such as nominal algebras and picalculus algebras. For Equational Systems we present an explicit construction of free algebras under reasonably general conditions. Term Equational Systems (TESs) are given by a more concrete, yet still abstract notion of equation. For TESs, we provide two means of equational reasoning: reasoning by deduction and reasoning by rewriting. For the reasoning by deduction, a set of sound deduction rules is given, but we do not have a general completeness result for it. However, we have an internal completeness result for TESs that admit free algebras. Together with this result, by analyzing the explicit construction of free algebras given by the theory of Equational Systems, one may synthesize a sound and complete equational reasoning by rewritng. Existing systems that arise as TESs include:
Indexable dictionaries and why dealing with bits in 2009.
Alessio Orlandi
(University of Pisa, IT)
Large scale data applications like search engines require very efficient
compression routines. Sometimes, even removing some bits per element
can result in a significant improvement. Random access to compressed
data also requires additional informations (and space).
Using the above scenario as a working example, we introduce (fully)
indexable dictionaries, succinct data structures for bitvectors
supporting
desirable operations in nearly minimal space.
We then illustrate some of the latest results concerning FIDs, both in
term
of theory and of real life applications.

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