Mean-field solution of the Hubbard model: the magnetic phase diagram

The present paper is based on our graduate lectures in condensed-matter physics. We found that the mean-field solution of the Hubbard model is an excellent tool to stimulate students' reflections towards the treatment of realistic magnetic interactions. We show by detailed analytical and numerical calculations how to find the mean-field solution of the model on a square lattice. We then interpret the physical implications of the ground-state magnetic phase diagram in terms of the electron density and the ratio between the Coulomb repulsion and the electron-structure bandwidth.Comment: 14 pages, 7 figure

There should not be any mystery: a comment on sampling issues in bibliometrics

A research unit wants to assess whether its publications tend to be more cited than academic publications in general. This research unit could be anything from a lonesome researcher to a national research council sponsoring thousands of researchers. The unit has access to the (inverted) percentile ranks of n of its publications: each publication has an associated real number between 0 and 100, which measures the percentage of publications in its reference set that receive at least as many citations as its own citation count. For instance, if the percentile rank of one publication is 10%, it means that 90% of publications in its reference set are less cited and 10% are cited at least as much. Now, say that the unit takes the mean of its n percentile ranks and reaches a value below 50%. Can the unit confidently conclude that it produces research that tends to be more cited than most publications? The article by Richard Williams and Lutz Bornmann (in press) proposes to answer this kind of question by relying on standard statistical procedures of significance testing and power analysis. I am deeply sympathetic to this proposal. I find, however, their exposition to be sometimes clouded in mystery. I suspect that many readers will therefore be unconvinced by their proposal. In this comment, I endeavor to make a clearer case for their general strategy. It should not be mysterious why this strategy is sound. By clarifying the case, I show that some technical decisions of Williams and Bornmann are mistakes (choosing a two-tailed instead of a one-tailed test), lead to less efficient estimates (choosing the t-statistic instead of simply relying on the mean) or are not as prudent as they should be (presuming a particular standard deviation). Before making these technical points, I start with a more conceptual issue: in the next section, I dispel some confusion regarding the notion of randomness in the presentation of the authors

Macrodynamics of economics: a bibliometric history

A history of specialties in economics since the late 1950s is constructed on the basis of a large corpus of documents from economics journals. The production of this history relies on a combination of algorithmic methods that avoid subjective assessments of the boundaries of specialties: bibliographic coupling, automated community detection in dynamic networks and text mining. these methods uncover a structuring of economics around recognizable specialties with some significant changes over the time-period covered (1956-2014). Among our results, especially noteworthy are (a) the clearcut existence of 10 families of specialties, (b) the disappearance in the late 1970s of a specialty focused on general economic theory, (c) the dispersal of the econometrics-centered specialty in the early 1990s and the ensuing importance of specific econometric methods for the identity of many specialties since the 1990s, (d) the low level of specialization of individual economists throughout the period in contrast to physicists as early as the late 1960s

Technical appendix to Macrodynamics of economics: a bibliometric history

A history of specialties in economics since the late 1950s is constructed on the basis of a large corpus of documents from economics journals. The production of this history relies on a combination of algorithmic methods that avoid subjective assessments of the boundaries of specialties: bibliographic coupling, automated community detection in dynamic networks, and text mining. These methods uncover a structuring of economics around recognizable specialties with some significant changes over the period covered (1956–2014). Among our results, especially noteworthy are (1) the clear-cut existence of ten families of specialties, (2) the disappearance in the late 1970s of a specialty focused on general economic theory, (3) the dispersal of the econometrics-centered specialty in the early 1990s and the ensuing importance of specific econometric methods for the identity of many specialties since the 1990s, and (4) the low level of specialization of individual economists throughout the period in contrast to physicists as early as the late 1960s

Consolidating Peace through PSSM Projects in Burundi

Small arms and light weapons have been a problem in Burundi due to the Burundi Civil War. This article focuses on the programs that (MAG) Mines Advisory Group and the Burundian police have instituted to reduce stockpiles and improve physical security and stockpile management in the country

Boire en se fiant uniquement à sa perception de la soif vs. boire de façon ad libitum : quelles différences dans le volume et le patron de consommation de liquide lors d’un exercice prolongé à vélo ?

Présentement, les termes boire selon les perceptions de la soif et boire de façon ad libitum sont utilisés de façon interchangeable dans la littérature, mais devraient-ils l’être? Cette recherche a pour but d’examiner les différences dans la façon dont les athlètes consomment des liquides pendant l’exercice lorsqu’on leur demande de boire selon leurs perceptions de la soif ou de façon ad libitum. Utilisant un protocole croisé où les traitements ont été randomisés et contrebalancés, 10 hommes (27 ± 4 ans) ont parcouru 120 km (48 ± 4 % de la puissance maximale, à 33 °C, avec une humidité relative de 40 %) à deux reprises, tout en buvant de l’eau selon leurs perceptions de la soif ou de façon ad libitum. Les participants ont réalisé les parcours à vélo en 222 ± 11 min (p = 0,29). Bien que la perte de masse corporelle à la fin de l’exercice (3,1 ± 1,2 (soif) vs. 2,8 ± 0,8 % (ad libitum), p = 0,38) et le volume total d’eau consommé (2714 ± 933 (soif) vs. 2815 ± 718 mL (ad libitum), p = 0,65) étaient similaires entre les expérimentations, la perception de la soif avant chaque gorgée ainsi que le volume d’eau consommé par gorgée étaient significativement plus élevés dans la condition soif en comparaison de la condition ad libitum (p < 0,01 pour les deux paramètres), tandis que le nombre total de gorgées était significativement plus faible dans la condition soif que la condition ad libitum (p = 0,004). L’effort perçu, la température rectale et le rythme cardiaque étaient tous significativement plus élevés avec la soif que la consommation ad libitum de liquide (p < 0,01 pour les trois paramètres). Ces résultats démontrent que les deux stratégies d’hydratation sont associées à des patrons de consommation différents, mais maintiennent également l’équilibre hydrique pendant un effort prolongé. Bien que la régulation de la fréquence cardiaque, de la température rectale et de la perception de l’effort était différente entre les deux conditions, ces différences peuvent être considérées comme étant, d’un point de vue clinique et pratique, triviales. Les termes boire à la soif et boire ad libitum peuvent donc être utilisés de façon interchangeable pour guider la consommation de liquide pendant un exercice prolongé

Causal Reasoning in Economics: A Selective Exploration of Semantic, Epistemic and Dynamical Aspects

Economists reason causally. Like many other scientists, they aim at formulating justified causal claims about their object of study. This thesis contributes to our understanding of how causal reasoning proceeds in economics. By using the research on the causes of unemployment as a case study, three questions are adressed. What are the meanings of causal claims? How can a causal claim be adequately supported by evidence? How are causal beliefs affected by incoming facts? In the process of answering these semantic, epistemic and dynamical questions, the richness of causal reasoning in economics is brought to the foreground. In the part on semantics, it is argued that the meaning of causal generalizations in policy-oriented economics is best captured by using the concept of ‘inferential relation’ rather than the one of ‘referential relation’. This part contributes to develop an inferentialist semantics by distinguishing among types of inferential relations. In the part on epistemology, it is argued that justification in sciences like economics often relies, and ought to rely, on evidential variety―i.e. the combination of evidence from multiple sources. An interpretation of independence among evidential sources is offered: two evidential sources are fully independent if and only if the reasons why they would be unreliable do not overlap. It is then shown that the variety-of-evidence thesis does not hold under extreme circumstances. The part on dynamics focuses on deviant-case study. A storyline is provided for what deviant-case study aims at and how it proceeds in what is characterized as an ‘eclectic science’

Les liseuses Cybook et Sony sous la loupe des bibliothécaires

Quarante-six professionnels, dont plus d’une quarantaine de bibliothécaires du réseau des bibliothèques de Montréal, ont participé à un projet pilote au cours de l’été 2010. Ce projet consistait à évaluer deux tablettes de lecture de livres numériques : le Cybook de la compagnie française Booken et la tablette tactile de la compagnie japonaise Sony Electronics Inc. Les bibliothécaires devaient participer à l’une des quatre rencontres de formation organisées au cours des deux premières semaines de juillet. La formation servait à présenter aux bibliothécaires l’expérience et son contexte, les appareils à évaluer, leurs logiciels de transfert, les principales fonctionnalités de chacun, des conseils sur le téléchargement et des sites gratuits et libres de droits recommandés pour télécharger des livres numériques. Lors de la présentation, la plupart des participants ont reçu un appareil de chacun des modèles. Quelques-uns n’ont reçu qu’un seul appareil; on leur demandait de se les échanger entre eux afin d’évaluer le plus possible les deux appareils, mais cela n’était pas obligatoire. Un questionnaire comportant plus de 24 questions, dont plusieurs étaient ouvertes afin de laisser le plus de place possible aux commentaires de la part des bibliothécaires, devait être complété pour le début de septembre 2010. Par la suite, des rencontres de groupes, s’échelonnant sur les deuxième et troisième semaines de septembre 2010, étaient organisées avec les participants, afin d’établir des recommandations sur les service