Examinando por Autor "Mejía, Daniel"
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Ítem Broken windows policing and crime: Evidence from 80 Colombian cities(Universidad EAFIT, 2021-09-01) Mejía, Daniel; Norza, Ervyn; Tobón Zapata, Santiago; Vanegas-Arias, Martín; Universidad de los Andes; Policía Nacional de Colombia; Universidad EAFIT; Universidad EAFITWe study the effects of broken windows policing on crime using geo-located crime and arrest reports for 80 Colombian cities. Broadly defined, broken windows policing consists of intensifying arrests—sometimes for minor offenses—to deter potential criminals. To estimate causal effects, we build grids of 200 × 200 meters over the urban perimeter of all cities and produce event studies to look at the effects of shocks in police activity in the periods to follow. We use spikes in the number of arrests with no warrant—which are more likely associated with unplanned police presence—as a proxy for shocks in broken windows policing. As expected, we observe an increase in crimes during the shock period, as each arrest implies at least one crime report. In the following periods, crimes decrease both in the place of the arrests and the surroundings. With many treated grids and many places exposed to spillovers, these effects add up. On aggregate, the crime reduction offsets the observed increase during the shock period. Direct effects are more immediate and precise at low crime grids, but beneficial spillovers seem more relevant at crime hot spots. The effects of broken windows policing circumscribe to cities with low or moderate organized crime, consistent with criminal organizations planning their activities more systematically than disorganized criminals.Ítem Comparison of FEM software for 2D heat transfer analysis in sheet metal laser cutting(2015) Mejía, Daniel; Moreno, Aitor; Barandiaran, Iñigo; Ruíz, Óscar; Universidad EAFIT. Departamento de Ingeniería Mecánica; Laboratorio CAD/CAM/CAEFinite Element Methods (FEM) have been used to simulate a variety of physical phenomena in the industrial manufacturing sector -- This paper addresses the simulation of the thermal properties in the metal sheet laser cutting -- A comparison of very well known FEM software is presented -- The results present small differences between the temperature distributions computed by the different softwareÍtem The Deterrent Effect of Surveillance Cameras on Crime(Universidad EAFIT, 2020-03-20) Tobón Zapata, Santiago; Gómez, Santiago; Mejía, Daniel; stobonz@eafit.edu.coFrom the US to Colombia to China, millions of public surveillance cameras are at the core of crime prevention strategies. Yet, we know little about the effects of surveillance cameras on criminal behavior, especially in developing economies. We study an installation program in Medellín and find t hat t he q uasi-random allocation of cameras led to a decrease in crimes and arrests. With no increase in the monitoring capacity and no chance to use camera footage in prosecution, these results suggest offenders were deterred rather than incapacitated. We test for spillovers and find no evidence of crime displacement or diffusion of benefits to surrounding locations.Ítem Hessian Eigenfunctions for Triangular Mesh Parameterization(SCITEPRESS, 2016) Mejía, Daniel; Ruíz, Oscar; Cadavid, Carlos A.; Universidad EAFIT. Departamento de Ingeniería Mecánica; Laboratorio CAD/CAM/CAEHessian Locally Linear Embedding (HLLE) is an algorithm that computes the nullspace of a Hessian functional H for Dimensionality Reduction (DR) of a sampled manifold M -- This article presents a variation of classic HLLE for parameterization of 3D triangular meses -- Contrary to classic HLLE which estimates local Hessian nullspaces, the proposed approach follows intuitive ideas from Differential Geometry where the local Hessian is estimated by quadratic interpolation and a partition of unity is used to join all neighborhoods -- In addition, local average triangle normals are used to estimate the tangent plane TxM at x ∈ M instead of PCA, resulting in local parameterizations which reflect better the geometry of the surface and perform better when the mesh presents sharp features -- A high frequency dataset (Brain) is used to test our algorithm resulting in a higher rate of success (96.63%) compared to classic HLLE (76.4%)Ítem Spectral-based mesh segmentation(Springer Paris, 2016) Mejía, Daniel; Cadavid, Carlos A.; Ruíz-Salguero, Óscar; Universidad EAFIT. Departamento de Ingeniería Mecánica; Laboratorio CAD/CAM/CAEIn design and manufacturing, mesh segmentation is required for FACE construction in boundary representation (BRep), which in turn is central for featurebased design, machining, parametric CAD and reverse engineering, among others -- Although mesh segmentation is dictated by geometry and topology, this article focuses on the topological aspect (graph spectrum), as we consider that this tool has not been fully exploited -- We preprocess the mesh to obtain a edgelength homogeneous triangle set and its Graph Laplacian is calculated -- We then produce a monotonically increasing permutation of the Fiedler vector (2nd eigenvector of Graph Laplacian) for encoding the connectivity among part feature submeshes -- Within the mutated vector, discontinuities larger than a threshold (interactively set by a human) determine the partition of the original mesh -- We present tests of our method on large complex meshes, which show results which mostly adjust to BRep FACE partition -- The achieved segmentations properly locate most manufacturing features, although it requires human interaction to avoid over segmentation -- Future work includes an iterative application of this algorithm to progressively sever features of the mesh left from previous submesh removalsÍtem Spectral-based vertex re-labeling for mesh segmentation(2015) Mejía, Daniel; Ruíz, Óscar E.; Cadavid, Carlos; Universidad EAFIT. Departamento de Ingeniería Mecánica; Laboratorio CAD/CAM/CAEMesh segmentation can be achieved by considering (a) geometric, (b) topologic or (c) a combination of geometric and topologic features on the surface Altough considering geometric characteristics would be relatively easy, our main intention is to keep the discussion on the topological aspect given that topology-based methods are foggy in their basic understanding impairing consistent application -- This article re-labels the vertices of the mesh based on the Fiedler vector (Laplacian 2nd eigenvector) for encoding the connectivity among part feature sub-meshes -- Second differences of such vector with respect to the re-labeling labeling are computed after a filter has been applied to determine the mesh partition -- The segmentation achieved by the proposed algorithm locates properly several topological features, provided the homogeneity of the triangular meshÍtem Triangular mesh parameterization with trimmed surfaces(Springer Verlag, 2015) Ruíz, Óscar E.; Mejía, Daniel; Cadavid, Carlos A.; Universidad EAFIT. Departamento de Ingeniería Mecánica; Laboratorio CAD/CAM/CAEGiven a 2manifold triangular mesh \(M \subset {\mathbb {R}}^3\), with border, a parameterization of \(M\) is a FACE or trimmed surface \(F=\{S,L_0,\ldots, L_m\}\) -- \(F\) is a connected subset or region of a parametric surface \(S\), bounded by a set of LOOPs \(L_0,\ldots ,L_m\) such that each \(L_i \subset S\) is a closed 1manifold having no intersection with the other \(L_j\) LOOPs -- The parametric surface \(S\) is a statistical fit of the mesh \(M\) -- \(L_0\) is the outermost LOOP bounding \(F\) and \(L_i\) is the LOOP of the ith hole in \(F\) (if any) -- The problem of parameterizing triangular meshes is relevant for reverse engineering, tool path planning, feature detection, redesign, etc -- Stateofart mesh procedures parameterize a rectangular mesh \(M\) -- To improve such procedures, we report here the implementation of an algorithm which parameterizes meshes \(M\) presenting holes and concavities -- We synthesize a parametric surface \(S \subset {\mathbb {R}}^3\) which approximates a superset of the mesh \(M\) -- Then, we compute a set of LOOPs trimming \(S\), and therefore completing the FACE \(F=\ {S,L_0,\ldots ,L_m\}\) -- Our algorithm gives satisfactory results for \(M\) having low Gaussian curvature (i.e., \(M\) being quasi-developable or developable) -- This assumption is a reasonable one, since \(M\) is the product of manifold segmentation preprocessing -- Our algorithm computes: (1) a manifold learning mapping \(\phi : M \rightarrow U \subset {\mathbb {R}}^2\), (2) an inverse mapping \(S: W \subset {\mathbb {R}}^2 \rightarrow {\mathbb {R}}^3\), with \ (W\) being a rectangular grid containing and surpassing \(U\) -- To compute \(\phi\) we test IsoMap, Laplacian Eigenmaps and Hessian local linear embedding (best results with HLLE) -- For the back mapping (NURBS) \(S\) the crucial step is to find a control polyhedron \(P\), which is an extrapolation of \(M\) -- We calculate \(P\) by extrapolating radial basis functions that interpolate points inside \(\phi (M)\) -- We successfully test our implementation with several datasets presenting concavities, holes, and are extremely nondevelopable -- Ongoing work is being devoted to manifold segmentation which facilitates mesh parameterizationÍtem The Unintended Consequences of the U.S. Adversarial Model in Latin American Crime(Universidad EAFIT, 2020-09-10) Zorro Medina, Angela; Acosta, Camilo; Mejía, Daniel; Universidad de Chicago; Universidad EAFIT; Universidad de los Andes; cacosta7@eafit.edu.coDuring the 1990s, Latin America experienced a criminal procedural revolution (LACPR) when approximately 70% of its countries abandoned their inquisitorial system and adopted the U.S. adversarial model. Following the LACPR, the region experienced a dramatic increase in crime, consolidating it as one of the most violent areas in the world. Despite previous empirical evidence indicating that procedural law affects criminal behavior, the effects of the LACPR continue highly unexplored. In this paper, we use the Latin American case to evaluate the impact of an adversarial reform on crime rates. Exploiting the quasi-experimental implementation of the reform in Colombia, we use an event study approach combined with differences-in-differences to estimate the reform’s effects on criminal activity. Despite the opposite incentives the reform created, we find an increase associated with the procedural transformation in overall crime rates (22%), violent crime (15%), and property crime (8%). We also observe a dramatic decrease in drug offenses associated with lower arrest rates. Our findings contribute to the literature on Latin American crime and the link between procedural law and criminal behavior.