568 research outputs found
Tractable Optimization Problems through Hypergraph-Based Structural Restrictions
Several variants of the Constraint Satisfaction Problem have been proposed
and investigated in the literature for modelling those scenarios where
solutions are associated with some given costs. Within these frameworks
computing an optimal solution is an NP-hard problem in general; yet, when
restricted over classes of instances whose constraint interactions can be
modelled via (nearly-)acyclic graphs, this problem is known to be solvable in
polynomial time. In this paper, larger classes of tractable instances are
singled out, by discussing solution approaches based on exploiting hypergraph
acyclicity and, more generally, structural decomposition methods, such as
(hyper)tree decompositions
Dagstuhl News January - December 2006
"Dagstuhl News" is a publication edited especially for the members of the Foundation "Informatikzentrum Schloss Dagstuhl" to thank them for their support. The News give a summary of the scientific work being done in Dagstuhl. Each Dagstuhl Seminar is presented by a small abstract describing the contents and scientific highlights of the seminar as well as the perspectives or challenges of the research topic
Parameterized Complexity of Binary CSP: Vertex Cover, Treedepth, and Related Parameters
We investigate the parameterized complexity of Binary CSP parameterized by the vertex cover number and the treedepth of the constraint graph, as well as by a selection of related modulator-based parameters. The main findings are as follows:
- Binary CSP parameterized by the vertex cover number is W[3]-complete. More generally, for every positive integer d, Binary CSP parameterized by the size of a modulator to a treedepth-d graph is W[2d+1]-complete. This provides a new family of natural problems that are complete for odd levels of the W-hierarchy.
- We introduce a new complexity class XSLP, defined so that Binary CSP parameterized by treedepth is complete for this class. We provide two equivalent characterizations of XSLP: the first one relates XSLP to a model of an alternating Turing machine with certain restrictions on conondeterminism and space complexity, while the second one links XSLP to the problem of model-checking first-order logic with suitably restricted universal quantification. Interestingly, the proof of the machine characterization of XSLP uses the concept of universal trees, which are prominently featured in the recent work on parity games.
- We describe a new complexity hierarchy sandwiched between the W-hierarchy and the A-hierarchy: For every odd t, we introduce a parameterized complexity class S[t] with W[t] ? S[t] ? A[t], defined using a parameter that interpolates between the vertex cover number and the treedepth. We expect that many of the studied classes will be useful in the future for pinpointing the complexity of various structural parameterizations of graph problems
Parameterized Complexity of Binary CSP: Vertex Cover, Treedepth, and Related Parameters
We investigate the parameterized complexity of Binary CSP parameterized by the vertex cover number and the treedepth of the constraint graph, as well as by a selection of related modulator-based parameters. The main findings are as follows: Binary CSP parameterized by the vertex cover number is W[3]-complete. More generally, for every positive integer d, Binary CSP parameterized by the size of a modulator to a treedepth-d graph is W[2d + 1]-complete. This provides a new family of natural problems that are complete for odd levels of the W-hierarchy. We introduce a new complexity class XSLP, defined so that Binary CSP parameterized by treedepth is complete for this class. We provide two equivalent characterizations of XSLP: the first one relates XSLP to a model of an alternating Turing machine with certain restrictions on conondeterminism and space complexity, while the second one links XSLP to the problem of model-checking first-order logic with suitably restricted universal quantification. Interestingly, the proof of the machine characterization of XSLP uses the concept of universal trees, which are prominently featured in the recent work on parity games. We describe a new complexity hierarchy sandwiched between the W-hierarchy and the A-hierarchy: For every odd t, we introduce a parameterized complexity class S[t] with W[t] ⊆ S[t] ⊆ A[t], defined using a parameter that interpolates between the vertex cover number and the treedepth. We expect that many of the studied classes will be useful in the future for pinpointing the complexity of various structural parameterizations of graph problems
Simulated penetration testing and mitigation analysis
Da Unternehmensnetzwerke und Internetdienste stetig komplexer werden, wird es immer schwieriger, installierte Programme, Schwachstellen und Sicherheitsprotokolle zu überblicken. Die Idee hinter simuliertem Penetrationstesten ist es, Informationen über ein Netzwerk in ein formales Modell zu transferiern und darin einen Angreifer zu simulieren. Diesem Modell fügen wir einen Verteidiger hinzu, der mittels eigener Aktionen versucht, die Fähigkeiten des Angreifers zu minimieren. Dieses zwei-Spieler Handlungsplanungsproblem nennen wir Stackelberg planning. Ziel ist es, Administratoren, Penetrationstestern und der Führungsebene dabei zu helfen, die Schwachstellen großer Netzwerke zu identifizieren und kosteneffiziente Gegenmaßnahmen vorzuschlagen. Wir schaffen in dieser Dissertation erstens die formalen und algorithmischen Grundlagen von Stackelberg planning. Indem wir dabei auf klassischen Planungsproblemen aufbauen, können wir von gut erforschten Heuristiken und anderen Techniken zur Analysebeschleunigung, z.B. symbolischer Suche, profitieren. Zweitens entwerfen wir einen Formalismus für Privilegien-Eskalation und demonstrieren die Anwendbarkeit unserer Simulation auf lokale Computernetzwerke. Drittens wenden wir unsere Simulation auf internetweite Szenarien an und untersuchen die Robustheit sowohl der E-Mail-Infrastruktur als auch von Webseiten. Viertens ermöglichen wir mittels webbasierter Benutzeroberflächen den leichten Zugang zu unseren Tools und Analyseergebnissen.As corporate networks and Internet services are becoming increasingly more complex, it is hard to keep an overview over all deployed software, their potential vulnerabilities, and all existing security protocols. Simulated penetration testing was proposed to extend regular penetration testing by transferring gathered information about a network into a formal model and simulate an attacker in this model. Having a formal model of a network enables us to add a defender trying to mitigate the capabilities of the attacker with their own actions. We name this two-player planning task Stackelberg planning. The goal behind this is to help administrators, penetration testing consultants, and the management level at finding weak spots of large computer infrastructure and suggesting cost-effective mitigations to lower the security risk. In this thesis, we first lay the formal and algorithmic foundations for Stackelberg planning tasks. By building it in a classical planning framework, we can benefit from well-studied heuristics, pruning techniques, and other approaches to speed up the search, for example symbolic search. Second, we design a theory for privilege escalation and demonstrate the applicability of our framework to local computer networks. Third, we apply our framework to Internet-wide scenarios by investigating the robustness of both the email infrastructure and the web. Fourth, we make our findings and our toolchain easily accessible via web-based user interfaces
Support at Home: Interventions to Enhance Life in Dementia (SHIELD) – evidence, development and evaluation of complex interventions
Background: Dementia is a national priority and this research addresses the Prime Minister’s commitment
to dementia research as demonstrated by his 2020 challenge and the new UK Dementia Research Institute.
In the UK > 800,000 older people have dementia. It has a major impact on the lives of people with
dementia themselves, on the lives of their family carers and on services, and costs the nation ÂŁ26B per
year. Pharmacological cures for dementias such as Alzheimer’s disease are not expected before 2025. If no
cure can be found, the ageing demographic will result in 2 million people living with dementia by 2050.
People with dementia lose much more than just their memory and their daily living skills; they can also lose
their independence, their dignity and status, their confidence and morale, and their roles both within the
family and beyond. They can be seen as a burden by society, by their families and even by themselves, and
may feel unable to contribute to society. This programme of research aims to find useful interventions to
improve the quality of life of people with dementia and their carers, and to better understand how people
with dementia can be supported at home and avoid being admitted to hospital.
Objectives: (1) To develop and evaluate the maintenance cognitive stimulation therapy (MCST) for people
with dementia; (2) to develop the Carer Supporter Programme (CSP), and to evaluate the CSP and
Remembering Yesterday, Caring Today (RYCT) for people with dementia both separately and together in
comparison with usual care; and (3) to develop a home treatment package (HTP) for dementia, to field test
the HTP in practice and to conduct an exploratory trial.
Methods: (1) The MCST programme was developed for people with dementia based on evidence and
qualitative work. A randomised controlled trial (RCT) [with a pilot study of MCST plus acetylcholinesterase
inhibitors (AChEIs)] compared MCST with cognitive stimulation therapy (CST) only. The MCST implementation
study conducted a trial of outreach compared with usual care, and assessed implementation in practice.
(2) The CSP was developed based on existing evidence and the engagement of carers of people with dementia. The RCT (with internal pilot) compared the CSP and reminiscence (RYCT), both separately and
in combination, with usual care. (3) A HTP for dementia, including the most promising interventions and
components, was developed by systematically reviewing the literature and qualitative studies including
consensus approaches. The HTP for dementia was evaluated in practice by conducting in-depth field testing.
Results: (1) Continuing MCST improved quality of life and improved cognition for those taking AChEIs.
It was also cost-effective. The CST implementation studies indicated that many staff will run CST groups
following a 1-day training course, but that outreach support helps staff go on to run maintenance groups
and may also improve staff sense of competence in dementia care. The study of CST in practice found no
change in cognition or quality of life at 8-month follow-up. (2) The CSP/RYCT study found no benefits for
family carers but improved quality of life for people with dementia. RYCT appeared beneficial for the
quality of life of people with dementia but at an excessively high cost. (3) Case management for people
with dementia reduces admissions to long-term care and reduces behavioural problems. In terms of
managing crises, staff suggested more costly interventions, carers liked education and support, and people
with dementia wanted family support, home adaptations and technology. The easy-to-use home treatment
manual was feasible in practice to help staff working in crisis teams to prevent hospital admissions for
people with dementia.
Limitations: Given constraints on time and funding, we were unable to compete the exploratory trial of
the HTP package or to conduct an economic evaluation.
Future research: To improve the care of people with dementia experiencing crises, a large-scale clinical
trial of the home treatment manual is needed.
Conclusion: There is an urgent need for effective psychosocial interventions for dementia. MCST improved
quality of life and was cost-effective, with benefits to cognition for those on AChEIs. MCST was feasible in
practice. Both CSP and RYCT improved the quality of life of people with dementia, but the overall costs
may be too high. The HTP was useful in practice but requires evaluation in a full trial. Dementia care
research may improve the lives of millions of people across the world.
Trial registrations: Current Controlled Trials ISRCTN26286067 (MCST), ISRCTN28793457 (MCST
implementation) and ISRCTN37956201 (CSP/RYCT).
Funding: This project was funded by the National Institute for Health Research (NIHR) Programme Grants
for Applied Research programme and will be published in full in Programme Grants for Applied Research;
Vol. 5, No. 5. See the NIHR Journals Library website for further project information
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