The Role of Normalization in the Belief Propagation Algorithm

An important part of problems in statistical physics and computer science can be expressed as the computation of marginal probabilities over a Markov Random Field. The belief propagation algorithm, which is an exact procedure to compute these marginals when the underlying graph is a tree, has gained its popularity as an efficient way to approximate them in the more general case. In this paper, we focus on an aspect of the algorithm that did not get that much attention in the literature, which is the effect of the normalization of the messages. We show in particular that, for a large class of normalization strategies, it is possible to focus only on belief convergence. Following this, we express the necessary and sufficient conditions for local stability of a fixed point in terms of the graph structure and the beliefs values at the fixed point. We also explicit some connexion between the normalization constants and the underlying Bethe Free Energy

Land tenurial systems and the adoption of Mucuna planted fallow in the derived savannas of West Africa:

In 1987, an improved resource management system that incorporates velvet bean (Mucuna pruriens var. utilis) to address soil fertility and weed (Imperata cylindrica) infestation was introduced to the small-scale farmers in a densely populated area of the derived savannas in Benin Republic (West Africa). Six years later, an adoption study was conducted to assess factors driving the adoption process. Four types of land tenure systems based on mode of access to land were identified: divided inheritance, purchasing, gifts, and sharecropping/renting. The first three provide long-term security over land, and together, they represent about 76 percent of the survey fields. Results from three variants of a probit model indicated that security over land was among the factors that significantly affect the adoption of the technology, with a high marginal effect on the probability of adoption, while gender did not have a significant effect. The most important determinant for adoption is the number of times a field is weeded during a cropping season (a proxy for the amount of labor required to tend a crop for better yields). High weeding requirements favorably affect the adoption of velvet bean only if farmers have full security on the degraded (weedy) land. The predominance of land tenure systems that provide secure property rights, namely the traditional acquisition of land through inheritance or gift mode and the gradual development of a land market, facilitated a quick spread of the Mucuna planted fallows in the study region.Watershed management., Water use India Citizen participation., Irrigation projects India., Gender, Property rights, Agricultural technology, Agricultural growth,

Local stability of Belief Propagation algorithm with multiple fixed points

A number of problems in statistical physics and computer science can be expressed as the computation of marginal probabilities over a Markov random field. Belief propagation, an iterative message-passing algorithm, computes exactly such marginals when the underlying graph is a tree. But it has gained its popularity as an efficient way to approximate them in the more general case, even if it can exhibits multiple fixed points and is not guaranteed to converge. In this paper, we express a new sufficient condition for local stability of a belief propagation fixed point in terms of the graph structure and the beliefs values at the fixed point. This gives credence to the usual understanding that Belief Propagation performs better on sparse graphs.Comment: arXiv admin note: substantial text overlap with arXiv:1101.417

HEALTH COSTS AND EXTERNALITIES OF PESTICIDE USE IN LOCUST AND GRASSHOPPER CONTROL IN THE SAHEL

To evaluate the economic cost of classical chemical control of locusts, externalities of chemical pesticides in the Sahel are estimated through a farmer survey. The costs of human health, losses in domestic animals and of destroying obsolete pesticides are estimated at $4 per treated ha for chemical control.Environmental Economics and Policy, Health Economics and Policy,

Pairwise MRF Calibration by Perturbation of the Bethe Reference Point

We investigate different ways of generating approximate solutions to the pairwise Markov random field (MRF) selection problem. We focus mainly on the inverse Ising problem, but discuss also the somewhat related inverse Gaussian problem because both types of MRF are suitable for inference tasks with the belief propagation algorithm (BP) under certain conditions. Our approach consists in to take a Bethe mean-field solution obtained with a maximum spanning tree (MST) of pairwise mutual information, referred to as the \emph{Bethe reference point}, for further perturbation procedures. We consider three different ways following this idea: in the first one, we select and calibrate iteratively the optimal links to be added starting from the Bethe reference point; the second one is based on the observation that the natural gradient can be computed analytically at the Bethe point; in the third one, assuming no local field and using low temperature expansion we develop a dual loop joint model based on a well chosen fundamental cycle basis. We indeed identify a subclass of planar models, which we refer to as \emph{Bethe-dual graph models}, having possibly many loops, but characterized by a singly connected dual factor graph, for which the partition function and the linear response can be computed exactly in respectively O(N) and $O(N^2)$ operations, thanks to a dual weight propagation (DWP) message passing procedure that we set up. When restricted to this subclass of models, the inverse Ising problem being convex, becomes tractable at any temperature. Experimental tests on various datasets with refined $L_0$ or $L_1$ regularization procedures indicate that these approaches may be competitive and useful alternatives to existing ones.Comment: 54 pages, 8 figure. section 5 and refs added in V

Du papegau au perroquet : antonomase et parodie: antonomase et parodie

This article intends to study the link between antonomasia and parody based upon the use of two words : « papegau » (popinjay) and « perroquet » (parrot). What does the substitution of one sign with the other actually mean and to what extent does the antonomasia have a parodic potential ? And lastly, is the parrot a preferred parodic figure ?Cette étude est née d’une interrogation : pourquoi le potentiel parodique dont dispose le psittacus n’a-t-il pas été utilisé davantage et plus tôt dans la littérature ? Pour esquisser quelques réponses, nous allons retisser un réseau de textes et de sens pour rendre compte du glissement sémantique ou de la métamorphose qui s’opère en passant du papegau à Parroquet et de Parroquet au perroquet. Pour ce faire, nous envisagerons de manière privilégiée un texte bref, inséré dans Le Chevalier Errant (1394) de Thomas de Saluces, dans lequel apparaît la première occurrence connue du mot « perroquet ». Il nous a semblé intéressant de voir si ce texte fonctionne comme un pivot dans la représentation de l’oiseau du fait du changement de signe et ce qu’un tel changement implique

Entre balbutiement et radotage : Enfance, répétition et parodie dans le roman arthurien du Moyen Âge tardif

Pourquoi en cet automne du Moyen Âge l’enfance apparaît-elle comme une voie privilégiée du renouvellement de l’écriture et dans le même temps comme un lieu de retour vers l’origine du roman ? On peut y lire une nostalgie du temps passé et des écrits passés qu’il faut conserver, un mal du pays littéraire d’antan. Écrire l’enfance, c’est aussi se ménager la possibilité d’adjoindre un petit supplément, combler le blanc, mais ce faisant d’introduire une distance amusée sur le matériau d’origine. L’enfance est le lieu des possibles narratifs et le lieu de leur renouvellement, le lieu privilégié où se conjoignent l’autrefois et l’avenir ; l’aventure se redéfinit alors comme un toujours déjà-là enfoui ou refoulé qu’il faut faire remonter à la surface. Cet article se propose de dessiner quelques perspectives et d’appréhender la question de la récriture parodique et de son rapport au schème et au thème de l’enfance du héros à partir de trois axes principaux : l’écriture contrapuntique et la contrefaçon, la métafiction, et enfin la mémoire et la répétition puisque tout se joue finalement dans une tension entre balbutiement et radotage.Why in the “fall” of the Middle Ages did childhood appear as a privileged path for the renewal of writing and at the same time as the site for a return to the origins of the novel? The phenomena can be seen as a nostalgia for the past, for writings that the age had decided must be conserved, a longing for a long-lost literary country. To write of childhood is also to nurse the possibility of adding something to it, to fill in a blank, but in doing so to introduce an amused distance to the original material. Childhood is a place of narrative possibilities and renewal, the privileged site to unite past and future; the adventure of childhood is redefined as a buried or repressed “always already there” that must be brought to the surface. This article intends to lay out a few perspectives and analyse the question of rewriting as parody and of its relation to the scheme and theme of the childhood of the hero. It sets out from three principal axes: contrapuntal writing and pirating, the metafiction, and memory and repetition, because in the end everything plays out in the tension between stammering (balbutiement) and rambling (radotage)

Biogas from agricultural waste : Turning unavailable residues into accessible resources

Moving from a fossil dependent to a fossil free economy requires increased energy production from renewable resources. This thesis discusses the utilization of agricultural waste streams, such as straw and manure, for biogas production. The first part of the research presented focuses on pretreatment of straw with the aim to reduce the handling issues concerning straw and improve the degradation of the material during anaerobic digestion. The second part concerns process design of agro-based biogas production plants with the aim to find process configurations and feedstocks that lead to high bioenergy yields. The effect of co-digesting a manure-rich stream with a carbohydrate-rich stream and the role of the degree of carbohydrate accessibility on the methane production is also discussed.Wheat straw is a problematic material to digest due to its high porosity which causes it to float and makes it hard to pump/feed. Mechanical pretreatments that applied higher shearing to the straw, such as pelletization and extrusion, led to reduced floating layers. Particle size reduction of wheat straw impacted the methane production rate below 3 mm but did not have an impact on the methane yield. The particle size was, however, not the only factor affecting the methane production rate. Hammer milled straw and extruded straw had a similar particle size but the degradation of extruded straw was faster. To increase the methane production rate, a shearing effect of the pretreatment may be more important. To solve only the handling issues of the straw, it may not be worth the high energy demand of those pretreatments.Wheat straw cannot be digested without the addition of nutrients. Co-digestion with manure or animal bedding is thereby a promising solution. By washing the animal bedding, it was possible to separate out the fibers and subject them to pretreatment with similar yields as pretreatment of wheat straw. Such a process design opened up for parallel production of biogas from the manure-fraction and fiber hydrolysate, and bioethanol from the steam pretreated fibers. Co-digestion of manure and readily available hydrolysate led to an increased initial lag phase and additional studies presented in this thesis showed that a too high degree of carbohydrate accessibility will increase the risk of process instability due to volatile fatty acids accumulation. Further, because part of the carbohydrates was diverted for yeast fermentation, the C/N ratio in the anaerobic digestion step became low. To solve this, cow manure and additional wheat straw were added to the production process. Energy balances and estimated energy demands over the process, in comparisons with other designs, showed that biofuel production was more energy efficient without co-production of ethanol. However, because of the recovery of lignin, there is a great potential of covering most of the energy demand by on-site steam production. Like so, the energy efficiency would much improve