Time in discrete agent-based models of socio-economic systems

We formulate the problem of computing time in discrete dynamical agent-based models in the context of socio-economic modeling. For such formulation, we outline a simple solution. This requires minimal extensions of the original untimed model. The proposed solution relies on the notion of agent-specific schedules of action and on two modeling assumptions. These are fulfilled by most models of pratical interest. For models for which stronger assumptions can be made, we discuss alternative formulations.Agent-based models, time.

Collective excitations of the QED vacuum

Using relativistic Green’s-function techniques we examined single-electron excitations from the occupied Dirac sea in the presence of strong external fields. The energies of these excited states are determined taking into account the electron-electron interaction. We also evaluate relativistic transition strengths incorporating retardation, which represents a direct measure of correlation effects. The shifts in excitation energies are computed to be lower than 0.5%, while the correlated transition strengths never deviate by more than 10% from their bare values. A major conclusion is that we found no evidence for collectivity in the electron-positron field around heavy and superheavy nuclei

Time in discrete agent-based models of socio-economic systems

URL des Documents de travail : http://centredeconomiesorbonne.univ-paris1.fr/bandeau-haut/documents-de-travail/Documents de travail du Centre d'Economie de la Sorbonne 2010.76 - ISSN : 1955-611XWe formulate the problem of computing time in discrete dynamical agent-based models in the context of socio-economic modeling. For such formulation, we outline a simple solution. This requires minimal extensions of the original untimed model. The proposed solution relies on the notion of agent-specific schedules of action and on two modeling assumptions. These are fulfilled by most models of pratical interest. For models for which stronger assumptions can be made, we discuss alternative formulations.On formule le problème du décompte du temps dans des modèles multi-agents à dynamique discrète dans le contexte de la modélisation socio-économique et on en donne une solution simple qui requiert une extension minimale du modèle initial

Nonlinear extensions of the Dirac equation and their implications in QED

We investigate the influence of additional nonlinear terms in the Dirac Lagrangian on strongly bound electron states in heavy and superheavy atoms. Upper bounds for the coupling constants are deduced by comparison with precision spectroscopy data in QED. We demonstrate that nonlinear interactions may cause significant modifications of electron binding energies in superheavy quasiatomic systems which would not be visible in ordinary atomic-physics measurements

The impact of uncertainty on optimal emission policies

We apply a computational framework for specifying and solving sequential decision problems to study the impact of three kinds of uncertainties on optimal emission policies in a stylized sequential emission problem.We find that uncertainties about the implementability of decisions on emission reductions (or increases) have a greater impact on optimal policies than uncertainties about the availability of effective emission reduction technologies and uncertainties about the implications of trespassing critical cumulated emission thresholds. The results show that uncertainties about the implementability of decisions on emission reductions (or increases) call for more precautionary policies. In other words, delaying emission reductions to the point in time when effective technologies will become available is suboptimal when these uncertainties are accounted for rigorously. By contrast, uncertainties about the implications of exceeding critical cumulated emission thresholds tend to make early emission reductions less rewarding

Katalyse und Katalysatoren zur Hydroformylierung langkettiger Olefine in überkritischem Kohlendioxid

Eine Prozessentwicklung berücksichtigt für katalytische Reaktionen und Reaktionsschritte auch Möglichkeiten, wie der Katalysator rezykliert und seine Leistungsfähigkeit gesteigert werden kann. Die vorliegende Arbeit befasste sich am Beispiel der Hydroformylierung von 1-Octen in überkritischem Kohlendioxid (scCO2) mit der Untersuchung verschiedener Ansätze, wie dieses realisiert werden kann. ScCO2 als Reaktionsmedium bietet den Vorteil, dass durch seine Verwendung das Phasenregime der normalerweise in einem heterogenen Gas-Flüssig-Zweiphasengemisch ablaufenden Hydroformylierung derart eingestellt werden kann, dass sie homogen in der scCO2-Phase verläuft. Damit gleichzeitig die Vorteile einer homogenen Katalyse bestehen bleiben, sind für die Entwicklung entsprechender Verfahren, bzw. Verfahrensschritte auch Daten über die Löslichkeit der Katalysatoren in scCO2 nötig. In dieser Arbeit wurden die Löslichkeiten von Kobaltkomplexen des Typs Co2(CO)6(Phosphan)2 in scCO2 sowie unter Bedingungen, die die Zusammensetzung unter realen Reaktionsbedingungen simulieren, bestimmt. Ausgehend von den Ergebnissen dieser Löslichkeitsuntersuchungen wurde ein Derivat dieses Komplextyps ausgewählt, mit dem die Recyclingfähigkeit in der Katalyse in einer systematischen Reihe von Wiederholungsexperimenten getestet wurde. Das Abscheiden des Katalysators vom Reaktionsgemisch erfolgte durch Änderung der Zustandsvariablen Druck, mit anschließender Mikrofiltration. Die Ergebnisse zeigen, dass ein Katalysatorrecycling nur dann zu akzeptablen Umsatz- und Selektivitätswerten führt, wenn der entsprechende Phosphanligand nachdosiert wird, deutbar als ein Leaching des Liganden in der Sequenz von Reaktion und Trennung. Aus diesem Grund wurden in dieser Arbeit des weiteren Katalysatorsysteme berücksichtigt, von denen eine erhöhte Resistenz gegen Leaching vermutet wird. Grundkonzept dieser Überlegungen ist die Verknüpfung von scCO2 als Reaktionsmedium mit dem SILP-Konzept für die Hydroformylierung mit Rhodiumkomplexen. Die Ergebnisse belegen zunächst, dass das Recyclingverhalten der untersuchten Rhodiumkatalysatoren bereits alleine durch die Anwendung von scCO2 als Solvens deutlich verbessert wird. Die Applikation des SILP-Konzepts, d. h. die Immobilisierung homogener Katalysatoren in einer flüssigen ionischen Matrix auf der Oberfläche fester Trägerpartikel ergab in den vorliegenden Untersuchungen deutliche Hinweise darauf, dass teilweises Rhodium-Leaching die Ursache für eine langsame Abnahme der Aktivität bei wiederholter Verwendung war. Daher wurde im Folgenden untersucht, ob die Leaching-Tendenz durch die Verankerung der Metallzentren über bidendale Donorfunktionen direkt auf einem anorganischen Trägermaterial minimierbar ist. Die Experimente zeigten in diesem Fall, dass über eine Reihe von 14 Wiederholungsversuchen kein Aktivitäts- oder Selektivitätsverlust zu beobachten war. Allerdings wurde durch die Immobiliserung eine deutliche Aktivitätsabnahme verursacht; so sank die TOF50 von 30.000 h-1 (beobachtet für die homogenen Analoga) auf immer noch akzeptable 700 h-1 für den immobilisierten Komplex

Vulnerability to Poverty and Vulnerability to Climate Change : Conceptual Framework, Measurement and Synergies in Policy

This paper attempts to compare the concepts and metrics related to vulnerability notion as used in the poverty literature with those in the filed of climate change. Such comparison could shed light on the understanding of the perceived and real differences between the two fields and also help to identify possible policy synergies between the climate change and poverty communities. The analysis shows that while vulnerability concepts in both the disciplines are defendable, broader policy relevant statements about vulnerability could be made if the analysis clearly identifies three primitives introduced in Ionescu et al. (2006) namely, the entity that is vulnerable, the stimulus due to which the entity is vulnerable, and the preference criteria on the outcome of concern. The analysis shows significant similarities between the two fields in terms of vulnerability measurement. The link between the vulnerability metrics in the two fields can be established through the introduction of sensitivity notion. The analysis also shows that the vulnerability metrics in both fields demand a stricter restriction (namely, complete reorder) on preference criteria on the outcome(s) of concern. The analysis identifies two issues that, if addressed, could create synergies between vulnerability assessments in the climate change and the poverty communities. First, the climate change community could benefit from exploring a notion analogous to that of poverty. In development policy, the notion of poverty enables one to recognise that there is a need to focus not only on people who are likely to become poor due to some exogenous input, but also on those who already are poor (and may become even poorer). Second, it would be interesting to explore the analogous concepts of mitigation and adaptive capacity in poverty research. Similar to their use in climate research and policy, the analysis of these concepts could lead to the more explicit consideration of the multidimensional nature of both causes and outcomes of poverty, as well as of the multiple time scales on which these occur.vulnerability, poverty, Climate change, Adaptation

Sequential decision problems, dependent types and generic solutions

We present a computer-checked generic implementation for solving finite horizon sequential decision problems. This is a wide class of problems, including intertemporal optimizations, knapsack, optimal bracketing, scheduling, etc. The implementation can handle time-step dependent control and state spaces, and monadic representations of uncertainty (such as stochastic, non-deterministic, fuzzy, or combinations thereof). This level of genericity is achievable in a programming language with dependent types (we have used both Idris and Agda). Dependent types are also the means that allow us to obtain a formalization and computer-checked proof of the central component of our implementation: Bellman’s principle of optimality and the associated backwards induction algorithm. The formalization clarifies certain aspects of backwards induction and, by making explicit notions such as viability and reachability, can serve as a starting point for a theory of controllability of monadic dynamical systems, commonly encountered in, e.g., climate impact research.Publisher PDFPeer reviewe

The impact of uncertainty on optimal emission policies

We apply a computational framework for specifying and solving sequential decision problems to study the impact of three kinds of uncertainties on optimal emission policies in a stylized sequential emission problem. We find that uncertainties about the implementability of decisions on emission reductions (or increases) have a greater impact on optimal policies than uncertainties about the availability of effective emission reduction technologies and uncertainties about the implications of trespassing critical cumulated emission thresholds. The results show that uncertainties about the implementability of decisions on emission reductions (or increases) call for more precautionary policies. In other words, delaying emission reductions to the point in time when effective technologies will become available is suboptimal when these uncertainties are accounted for rigorously. By contrast, uncertainties about the implications of exceeding critical cumulated emission thresholds tend to make early emission reductions less rewarding