Free fermionic propagators on a lattice

A method used recently to obtain a formalism for classical fields with non-local actions preserving chiral symmetry and uniqueness of fermion fields yields a discrete version of Huygens' principle with free discrete propagators that recover their continuum forms in certain limit.Comment: LaTex document, 13 pages, 1 figure. Minor changes, two references adde

New Results for the MAP Problem in Bayesian Networks

This paper presents new results for the (partial) maximum a posteriori (MAP) problem in Bayesian networks, which is the problem of querying the most probable state configuration of some of the network variables given evidence. First, it is demonstrated that the problem remains hard even in networks with very simple topology, such as binary polytrees and simple trees (including the Naive Bayes structure). Such proofs extend previous complexity results for the problem. Inapproximability results are also derived in the case of trees if the number of states per variable is not bounded. Although the problem is shown to be hard and inapproximable even in very simple scenarios, a new exact algorithm is described that is empirically fast in networks of bounded treewidth and bounded number of states per variable. The same algorithm is used as basis of a Fully Polynomial Time Approximation Scheme for MAP under such assumptions. Approximation schemes were generally thought to be impossible for this problem, but we show otherwise for classes of networks that are important in practice. The algorithms are extensively tested using some well-known networks as well as random generated cases to show their effectiveness.Comment: A couple of typos were fixed, as well as the notation in part of section 4, which was misleading. Theoretical and empirical results have not change

M+D: conceptual guidelines for compiling a materials library

This article proposes to present a study conducted by the Raw Materials research group, the results of which comprise the conceptual guidelines for compiling an M+D material library. The study includes the topic, materials and design taking the impact of the changes that came into being in the post industrial era on project methodologies and the search for information regarding materials. Taking into account the importance and complexity that these relationships have taken on currently, we have studied the issue of materials based on Manzini (1983) and Ashby and Johnson (2002). Afterward different databases and materials libraries located in the Brazil, the United States, France and Italy geared toward design professionals and students were analyzed to understand what information and means of access to them were available. The project methodologies were approached based on Löbach (1991), Bürdeck (1994), Schulmann (1994), Baxter (1998), Dantas (1998 and 2005) and Papanek (1995 and 2000). This study sought to identify the key elements of the role of materials in the project process today, to serve as a parameter for the analysis of the models studied. A comparative analysis of the models investigated enabled identification of positive and negative aspects to adapt to the needs previously mentioned and identify conceptual guidelines for compiling a collection of materials for use in design projects. Keywords: Design, Materials, Project Methodology, Library</p

Searching for Bayesian Network Structures in the Space of Restricted Acyclic Partially Directed Graphs

Although many algorithms have been designed to construct Bayesian network structures using different approaches and principles, they all employ only two methods: those based on independence criteria, and those based on a scoring function and a search procedure (although some methods combine the two). Within the score+search paradigm, the dominant approach uses local search methods in the space of directed acyclic graphs (DAGs), where the usual choices for defining the elementary modifications (local changes) that can be applied are arc addition, arc deletion, and arc reversal. In this paper, we propose a new local search method that uses a different search space, and which takes account of the concept of equivalence between network structures: restricted acyclic partially directed graphs (RPDAGs). In this way, the number of different configurations of the search space is reduced, thus improving efficiency. Moreover, although the final result must necessarily be a local optimum given the nature of the search method, the topology of the new search space, which avoids making early decisions about the directions of the arcs, may help to find better local optima than those obtained by searching in the DAG space. Detailed results of the evaluation of the proposed search method on several test problems, including the well-known Alarm Monitoring System, are also presented