Cluster algebras, quiver representations and triangulated categories

This is an introduction to some aspects of Fomin-Zelevinsky’s cluster algebras and their links with the representation theory of quivers and with Calabi-Yau triangulated categories. It is based on lectures given by the author at summer schools held in 2006 (Bavaria)and 2008 (Jerusalem). In addition to by now classical material, we present the outline of a proof of the periodicity conjecture for pairs of Dynkin diagrams (details will appear elsewhere) and recent results on the interpretation of mutations as derived equivalences.

Plot Suite

Una Plot Suite és una aplicació web que permet localitzar plots d’una Base de Dades a partir de formularis. S’obtindran taules on apareixeran els plots amb les seves característiques i es podrà obtenir còpies dels plots sol·licitats. Gràcies al seu disseny es podran afegir nous plots a la Base de Dades i fins i tot modificar l’estructura d’una manera molt intuïtiva.

Contracting Institutions and Development

The quality of contracting institutions has been thought to be of second-order importance next to the impact that good property rights institutions can have on long-run growth. Using a large range of proxies for each type of institution, we find a robust negative link between the quality of contracting institutions and long-run growth when we condition on property rights and a number of additional macroeconomic variables. Although the result remains something of a puzzle, we present evidence which suggests that only when property rights institutions are good do contracting institutions appear also to be good for development. Good contracting institutions can reduce long-run growth when property rights are not secured, presumably because the gains from the (costly) contracting institutions cannot be realised. This suggests that contracting institutions can benefit growth, and that the sequence of institutional change can matter.

The Expenditure Impacts of Individual Higher Education Institutions (HEIs) and their Students on the Welsh Economy: Homogeneity or Heterogeneity?

This paper replicates the analysis of Scottish HEIs in Hermannsson et al (2010a) for the case of Wales in order to provide a self-contained analysis that is readily accessible by those whose primary concern is with the regional impacts of Welsh HEIs. When we treat each of the twelve Higher Education Institutions (HEIs) that existed in Wales in 2006 as separate sectors in conventional input-output analysis, their expenditure impacts per unit of final demand appear rather homogenous, with the apparent heterogeneity of their overall impacts being primarily driven by scale. However, a disaggregation of their income by source reveals considerable variation in their dependence upon funding from the devolved Welsh Assembly Government and their ability to draw in income/funding from external sources. Acknowledging the binding budget constraint of the Welsh Assembly Government and deriving balanced expenditure multipliers reveals large differences in the net-expenditure impact of HEIs upon the Welsh economy, with the source of variation being the origin of income. Applying a novel treatment of student expenditure impacts, identifying the amount of exogenous spending per student, modifies the heterogeneity of the overall expenditure impacts. On balance this suggests that the impacts of impending budget cut-backs will be quite different by institution depending on their sensitivity to public funding. However, predicting the outcome of budget cutbacks at the margin is problematic for reasons that we identify.

The Expenditure Impacts of London-based Individual Higher Education Institutions (HEIs) and their Students on the Economy of England: Homogeneity or Heterogeneity?

This paper replicates the analysis of Scottish HEIs in Hermannsson et al (2010a) to identify the impact of London-based HEIs on the English economy in order to provide a self-contained analysis that is readily accessible by those whose primary concern is with the regional impacts of London HEIs. When we treat each of the 38 London-based Higher Education Institutions (HEIs) that existed in England in 2006 as separate sectors in conventional input-output analysis, their expenditure impacts per unit of final demand appear rather homogenous (though less so than HEIs in Wales and Scotland), with the apparent heterogeneity of their overall impacts being primarily driven by scale. However, a disaggregation of their income by source reveals considerable variation in their dependence upon general public funding and their ability to draw in income/funding from external sources. Acknowledging the possible alternative uses of the public funding and deriving balanced expenditure multipliers reveals large differences in the net-expenditure impact of London HEIs upon the English economy, with the source of variation being the origin of income. Applying a novel treatment of student expenditure impacts, identifying the amount of exogenous spending per student, modifies the heterogeneity of the overall expenditure impacts. On balance this suggests that the impacts of impending budget cut-backs will be quite different by institution depending on their sensitivity to public funding. However, predicting the outcome of budget cutbacks at the margin is problematic for reasons that we identify.

Minu, Startu and all that:- Pitfalls in estimating the sensitivity of a worker’s wage to aggregate unemployment

In this paper we show that the inclusion of unemployment-tenure interaction variates in Mincer wage equations is subject to serious pitfalls. These variates were designed to test whether or not the sensitivity to the business cycle of a worker’s wage varies according to her tenure. We show that three canonical variates used in the literature - the minimum unemployment rate during a worker’s time at the firm(min u), the unemployment rate at the start of her tenure(Su) and the current unemployment rate interacted with a new hire dummy(δu) - can all be significant and "correctly" signed even when each worker in the firm receives the same wage, regardless of tenure (equal treatment). In matched data the problem can be resolved by the inclusion in the panel of firm-year interaction dummies. In unmatched data where this is not possible, we propose a solution for min u and Su based on Solon, Barsky and Parker’s(1994) two step method. This method is sub-optimal because it ignores a large amount of cross tenure variation in average wages and is only valid when the scaled covariances of firm wages and firm employment are acyclical. Unfortunately δu cannot be identified in unmatched data because a differential wage response to unemployment of new hires and incumbents will appear under both equal treatment and unequal treatment.

The Expenditure Impacts of Individual Higher Education Institutions (HEIs) and their Students on the Northern Irish Economy: Homogeneity or Heterogeneity?

This paper replicates the analysis of Scottish HEIs in Hermannsson et al (2010a) for the case of Northern Ireland in order to provide a self-contained analysis that is readily accessible by those whose primary concern is with the regional impacts of Northern-Irish HEIs. When we treat each of the four Higher Education Institutions (HEIs) that existed in Northern Ireland in 2006 as separate sectors in conventional input-output analysis, their expenditure impacts per unit of final demand appear rather homogenous, with the apparent heterogeneity of their overall impacts being primarily driven by scale. However, a disaggregation of their income by source reveals considerable variation in their dependence upon funding from the devolved Assembly and their ability to draw in income/funding from external sources. Acknowledging the binding budget constraint of the Northern Ireland Assembly and deriving balanced expenditure multipliers reveals large differences in the netexpenditure impact of HEIs upon the Northern Irish economy, with the source of variation being the origin of income. Applying a novel treatment of student expenditure impacts, identifying the amount of exogenous spending per student, modifies the heterogeneity of the overall expenditure impacts. On balance this suggests that the impacts of impending budget cut-backs will be quite different by institution depending on their sensitivity to public funding. However, predicting the outcome of budget cutbacks at the margin is problematic for reasons that we identify.

Optimal taxation and the skill premium

The stylized facts suggest a negative relationship between tax progressivity and the skill premium from the early 1960s until the early 1990s, and a positive one thereafter. They also generally imply rising tax progressivity, except for the 1980s. In this paper, we ask whether optimal tax policy is consistent with these observations, taking into account the demographic and technological factors that have also affected the skill premium. To this end, we construct a dynamic general equilibrium model in which the skill premium and the progressivity of the tax system are endogenously determined, with the latter being optimally chosen by a benevolent government. We find that optimal policy delivers both a progressive tax system and model predictions which are generally consistent, except for the 1980s, with the stylized facts relating to the skill premium and progressivity. To capture the patterns in the data over the 1980s requires that we adopt a government policy which is biased towards the interests of skilled agents. Thus, in addition to demographic and technological factors, changes in the preferences of policy-makers appear to be a potentially important factor in determining the evolution of the observed skill premium.

Does readiness to change predict subsequent alcohol consumption in medical inpatients with unhealthy alcohol use?

We studied whether readiness to change predicts alcohol consumption (drinks per day) 3 months later in 267 medical inpatients with unhealthy alcohol use. We used 3 readiness to change measures: a 1 to 10 visual analog scale (VAS) and two factors of the Stages of Change Readiness and Treatment Eagerness Scale: Perception of Problems (PP) and Taking Action (TA). Subjects with the highest level of VAS-measured readiness consumed significantly fewer drinks 3 months later [Incidence rate ratio (IRR) and 95% confidence interval (CI): 0.57 (0.36, 0.91) highest vs. lowest tertile]. Greater PP was associated with more drinking [IRR (95%CI): 1.94 (1.02, 3.68) third vs. lowest quartile]. Greater TA scores were associated with less drinking [IRR (95%CI): 0.42 (0.23, 0.78) highest vs. lowest quartile]. Perception of Problems' association with more drinking may reflect severity rather than an aspect of readiness associated with ability to change; high levels of Taking Action appear to predict less drinking. Although assessing readiness to change may have clinical utility, assessing the patient's planned actions may have more predictive value for future improvement in alcohol consumption.

Social evolution in the sweat bee Halictus scabiosae

The evolution of eusociality is one of the major evolutionary transitions of life on earth. For investigating the conditions and processes that are central to the origin of such integrated social organization, it is best to study organisms in which individuals have retained some flexibility in their reproductive strategies. Halictid bees are especially well suited as model organisms, because they show huge variation in social systems, both within and between species. In this thesis, I investigated female reproductive strategies in the primitively eusocial bee Halictus scabiosae, with a focus on the role of helpers, in order to get insight into the mechanisms governing the evolution and maintenance of eusociality. This species produces two broods per year. The females from the first brood can stay in the natal nest to help raise a second brood of males and gynes that become the next-generation foundresses in spring. We first compared the morphology of females from the two broods, as well as the nutrition they receive as larvae. Then we conducted a helper- removal experiment in the field to quantify the effects of the presence of helpers on colony survival and productivity. Finally, we reconstructed pedigree relationships of individuals using microsatellite markers in order to detect who reproduces in the nest and how much individuals drift between nests. We found that first brood females had a uniformly small size and low fat reserves, which may be caused by the restricted pollen and nectar provisions on which they develop. Colony survival and productivity was increased by the presence of a single helper, but the effect was small and mostly limited to small colonies. By inferring parentage within and across colonies, we could determine that females from the first brood rarely reproduce in their natal nests. However, foundresses are frequently replaced, and foundresses and females from the first brood occasionally move to and reproduce in foreign colonies. As a result, colonies often contain offspring from unrelated individuals, and the relatedness of females to the brood they rear is low. Overall, this thesis shows that the reproductive system of H. scabiosae is highly flexible. The production of helpers in the first brood is important for colony success and productivity, but there is a high colony failure rate and part of the first brood females drift and reproduce in foreign nests. Both foundresses and helpers appear to be constrained by harsh environmental conditions or social factors limiting reproduction and independent colony founding. - L'origine des insectes sociaux est un domaine fascinant pour la recherche. Pour comprendre les mécanismes et les conditions qui sont nécessaires pour l'évolution et le maintien de la vie en société, il est judicieux d'étudier des sociétés primitives d'insectes, où toutes les femelles ont conservé la capacité de se reproduire, même si leur rôle comportemental dans la colonie est d'aider sans se reproduire. Une des familles d'abeilles, les halictes, est idéale pour cette sorte de recherche, en raison de la grande variabilité dans leur comportement social. Dans cette thèse, j'ai étudié les stratégies reproductives des femelles de Halictus scabiosae pour mieux comprendre les mécanismes qui influencent l'évolution de la vie en société. Cette espèce produit deux cohortes de couvain par année. Les femelles du premier couvain restent souvent dans leur nid natal pour aider à élever le deuxième couvain, tandis que les femelles du deuxième couvain s'accouplent et hibernent pour devenir les nouvelles fondatrices au printemps suivant. Nous avons d'abord comparé la morphologie des femelles issues des deux couvains ainsi que leur nutrition au stade de larve. Puis, dans une expérience sur le terrain, nous avons quantifié l'apport d'une ouvrière pour la survie et la productivité de la colonie. Finalement, nous avons reconstruit des pedigrees en utilisant des marqueurs génétiques, pour savoir qui se reproduit dans la colonie et combien d'individus migrent entre colonies. Les résultats montrent que les femelles du premier couvain sont uniformément plus petites et plus maigres, ce qui indique que les fondatrices réduisent les provisions de nourriture pour leur premier couvain afin de les inciter à aider dans le nid au lieu de se reproduire indépendamment. Dans l'expérience sur le terrain, la survie et la productivité de la colonie augmentaient avec la présence d'une ouvrière additionnelle, mais l'effet était petit et limité aux petites colonies. Par la reconstruction de pedigrees, nous pouvions constater que les femelles du premier couvain pondent rarement dans leurs nids natals. Les fondatrices cependant sont souvent remplacées en cours de saison, et migrent fréquemment entre nids, tandis que les femelles du premier couvain pondent parfois des oeufs dans des nids étrangers. De ce fait, les colonies contiennent souvent des descendants d'individus étrangers, et la parenté génétique entre les femelles et le deuxième couvain est basse. Cette thèse démontre que le système reproductif de H. scabiosae est très flexible. La production d'ouvrières est importante pour la survie de la colonie et sa productivité, mais le taux d'échec est élevé et une partie des femelles du premier couvain migrent et pondent dans une colonie étrangère. Autant les fondatrices que les ouvrières semblent être contraintes par des conditions environnementales ou sociales qui limitent la reproduction et les nouvelles fondations de colonie. - Die Entstehung von sozialen Lebensformen ist eines der wichtigsten Entwicklungen in der Geschichte des Lebens. Um die Bedingungen oder Prozesse zu verstehen, welche bei der Entstehung und dem Erhalt von sozialen Merkmalen wichtig sind, sollte man Lebewesen untersuchen, welche je nach Umwelteinflüßen ihr soziales Verhalten flexibel ändern können. Furchenbienen (Halictidae) gehören dazu. Diese weisen nämlich ein breites Spektrum verschiedener sozialer Organisationsformen auf, oftmals sogar innerhalb der einzelnen Arten. In meiner Doktorarbeit befasste ich mich mit den Fortpflanzungsstrategien der Weibchen der Skabiosen-Furchenbiene Halictus scabiosae. Diese Art produziert zwei Brüten pro Jahr. Die Weibchen der ersten Brut bleiben dabei meist als Arbeiterinnen in ihrem Geburtsnest, wohingegen die Weibchen der zweiten Brut nach der Paarung überwintern, um im nächsten Frühling neue Kolonien zu gründen. In einem ersten Schritt verglichen wir die beiden Brüten bezüglich der Grösse und der Fettreserven der Weibchen sowie der Pollen-Nektar-Vorräte für die Larven. Dann bestimmten wir in einem Feldexperiment, wieviel eine zusätzliche Arbeiterin zum Überleben und zur Produktiviät der Kolonie beiträgt. Schliesslich ermittelten wir durch genetische Tests die Verwandtschaftsbeziehungen zwischen den Bienen, um herauszufinden, wer in den Kolonien tatsächlich die Eier legt und ob und wieviel die Bienen zwischen verschiedenen Nestern wandern. Wir stellten fest, dass die Weibchen von der ersten Brut einheitlich kleiner sind und weniger Fettreserven besitzen. Das weist daraufhin, dass die Nestgründerin die erste Brut unterernährt, um die Wahrscheinlichkeit zu erhöhen, dass diese Weibchen als Arbeiterinnen im Nest bleiben anstatt sich unabhängig fortzupflanzen. Schon eine einzelne zusätzliche Arbeiterin verbesserte die Überlebenschancen und Produktivität der Kolonie, der Effekt war allerdings klein und auf kleine Kolonien beschränkt. Die Verwandtschaftsanalysen zeigten, dass die Arbeiterinnen nur sehr selten ein Ei in ihr Geburtsnest legen. Erstaunlicherweise wanderten die Nestgründerinnen oft zwischen verschiedenen Nestern. Einige Weibchen der ersten Brut wanderten auch in ein fremdes Nest und produzierten dort Nachkommen. Diese Doktorarbeit zeigt, dass die Fortpflanzungsstrategien der Skabiosen-Furchenbiene tatsächlich sehr flexibel sind. Die Anwesenheit von Arbeiterinnen ist wichtig für das Überleben und die Produktivität der Kolonie. Die Misserfolgsraten bleiben jedoch hoch, und ein Teil der Weibchen der ersten Brut pflanzt sich in fremden Nestern fort. Sowohl die Nestgründerinnen als auch die Weibchen der ersten Brut scheinen durch Umweltsbedingungen oder durch soziale Faktoren in der Wahl ihrer Fortpflanzungs¬strategie eingeschränkt zu sein.

Union Mediation and Adaptation to Reciprocal Loyalty Arrangements

This study assesses the industrial relations application of the „loyalty-exit-voice‟ proposition. The loyalty concept is linked to reciprocal employer-employee arrangements and examined as a job attribute in a vignette questionnaire distributed to low and medium-skilled employees. The responses provided by employees in three European countries indicate that reciprocal loyalty arrangements, which involve the exchange of higher effort for job security, are one of the most desirable job attributes. This attribute exerts a higher impact on the job evaluations provided by unionised workers, compared to their non-union counterparts. This pattern is robust to a number of methodological considerations. It appears to be an outcome of adaptation to union mediated cooperation. Overall the evidence suggests that the loyalty-job evaluation profiles of unionised workers are receptive to repeated interaction and negative shocks, such as unemployment experience. This is not the case for the non-union workers. Finally, unionised workers appear to „voice‟ a lower job satisfaction, but exhibit low „exit‟ intentions, compared to the non-unionised labour.

Prise en charge de l'ulcère de jambe d'origine veineuse [Management of venous leg ulcers].

Leg ulcers are a major health and economic problem especially in elderly. More than 70% are associated with venous disease. Compressive therapy is the most effective treatment but bandages are often poorly tolerated and well trained nurses are required to apply them effectively. In recent years, the VAC system (vacuum assisted closure) has profoundly changed the wound healing approach. The objective is now to regenerate the tissues and not to replace them with skin grafts which give uncertain results. The other important challenge is to prevent recurrences. New pharmacologic treatments acting on microcirculation and hemostasis would probably appear in the near future opening new therapeutic perspectives.

Moral Hazard with Counterfeit Signals

In many moral hazard problems, the principal evaluates the agent's performance based on signals which the agent may suppress and replace with counterfeits. This form of fraud may affect the design of optimal contracts drastically, leading to complete market failure in extreme cases. I show that in optimal contracts, the principal deters all fraud, and does so by two complementary mechanisms. First, the principal punishes signals that are suspicious, i.e. appear counterfeit. Second, the principal is lenient on bad signals that the agent could suppress, but does not.

Aid and Development: Issues and Reflections

The paper has three main sections. The first is a review of two particular propositions which appear in Dambisa Moyo’s 2009 book Dead Aid which were not subjected to rigorous analysis in the reviews which appeared following its publication. The finding is that neither proposition survives serious scrutiny – that aid is responsible for most of sub-Saharan Africa’s economic woes and that the international bond market represents a viable alternative to foreign aid for the finance of development-oriented investment. The second questions some of the characteristics and uses of the World Bank’s Country Policy and Institutional Assessment (CPIA), particularly focussing on the use of an essentially ordinal measure in cardinal applications. The third subjects the UK Department for International Development’s Needs-Effectiveness Index to critical review, concluding that further consideration of its attributes is necessary.

Fast numerical algorithms for the computation of invariant tori in Hamiltonian systems

In this paper, we develop numerical algorithms that use small requirements of storage and operations for the computation of invariant tori in Hamiltonian systems (exact symplectic maps and Hamiltonian vector fields). The algorithms are based on the parameterization method and follow closely the proof of the KAM theorem given in [LGJV05] and [FLS07]. They essentially consist in solving a functional equation satisfied by the invariant tori by using a Newton method. Using some geometric identities, it is possible to perform a Newton step using little storage and few operations. In this paper we focus on the numerical issues of the algorithms (speed, storage and stability) and we refer to the mentioned papers for the rigorous results. We show how to compute efficiently both maximal invariant tori and whiskered tori, together with the associated invariant stable and unstable manifolds of whiskered tori. Moreover, we present fast algorithms for the iteration of the quasi-periodic cocycles and the computation of the invariant bundles, which is a preliminary step for the computation of invariant whiskered tori. Since quasi-periodic cocycles appear in other contexts, this section may be of independent interest. The numerical methods presented here allow to compute in a unified way primary and secondary invariant KAM tori. Secondary tori are invariant tori which can be contracted to a periodic orbit. We present some preliminary results that ensure that the methods are indeed implementable and fast. We postpone to a future paper optimized implementations and results on the breakdown of invariant tori.