95 research outputs found
Discretization of the 3D Monge-Ampere operator, between Wide Stencils and Power Diagrams
We introduce a monotone (degenerate elliptic) discretization of the
Monge-Ampere operator, on domains discretized on cartesian grids. The scheme is
consistent provided the solution hessian condition number is uniformly bounded.
Our approach enjoys the simplicity of the Wide Stencil method, but
significantly improves its accuracy using ideas from discretizations of optimal
transport based on power diagrams. We establish the global convergence of a
damped Newton solver for the discrete system of equations. Numerical
experiments, in three dimensions, illustrate the scheme efficiency
Adaptive, Anisotropic and Hierarchical cones of Discrete Convex functions
We address the discretization of optimization problems posed on the cone of
convex functions, motivated in particular by the principal agent problem in
economics, which models the impact of monopoly on product quality. Consider a
two dimensional domain, sampled on a grid of N points. We show that the cone of
restrictions to the grid of convex functions is in general characterized by N^2
linear inequalities; a direct computational use of this description therefore
has a prohibitive complexity. We thus introduce a hierarchy of sub-cones of
discrete convex functions, associated to stencils which can be adaptively,
locally, and anisotropically refined. Numerical experiments optimize the
accuracy/complexity tradeoff through the use of a-posteriori stencil refinement
strategies.Comment: 35 pages, 11 figures. (Second version fixes a small bug in Lemma 3.2.
Modifications are anecdotic.
Anisotropic Fast-Marching on cartesian grids using Lattice Basis Reduction
We introduce a modification of the Fast Marching Algorithm, which solves the
generalized eikonal equation associated to an arbitrary continuous riemannian
metric, on a two or three dimensional domain. The algorithm has a logarithmic
complexity in the maximum anisotropy ratio of the riemannian metric, which
allows to handle extreme anisotropies for a reduced numerical cost. We prove
the consistence of the algorithm, and illustrate its efficiency by numerical
experiments. The algorithm relies on the computation at each grid point of a
special system of coordinates: a reduced basis of the cartesian grid, with
respect to the symmetric positive definite matrix encoding the desired
anisotropy at this point.Comment: 28 pages, 12 figure
Automatic differentiation of non-holonomic fast marching for computing most threatening trajectories under sensors surveillance
We consider a two player game, where a first player has to install a
surveillance system within an admissible region. The second player needs to
enter the the monitored area, visit a target region, and then leave the area,
while minimizing his overall probability of detection. Both players know the
target region, and the second player knows the surveillance installation
details.Optimal trajectories for the second player are computed using a
recently developed variant of the fast marching algorithm, which takes into
account curvature constraints modeling the second player vehicle
maneuverability. The surveillance system optimization leverages a reverse-mode
semi-automatic differentiation procedure, estimating the gradient of the value
function related to the sensor location in time N log N
Minimal geodesics along volume preserving maps, through semi-discrete optimal transport
We introduce a numerical method for extracting minimal geodesics along the
group of volume preserving maps, equipped with the L2 metric, which as observed
by Arnold solve Euler's equations of inviscid incompressible fluids. The method
relies on the generalized polar decomposition of Brenier, numerically
implemented through semi-discrete optimal transport. It is robust enough to
extract non-classical, multi-valued solutions of Euler's equations, for which
the flow dimension is higher than the domain dimension, a striking and
unavoidable consequence of this model. Our convergence results encompass this
generalized model, and our numerical experiments illustrate it for the first
time in two space dimensions.Comment: 21 pages, 9 figure
Monotone and Consistent discretization of the Monge-Ampere operator
We introduce a novel discretization of the Monge-Ampere operator,
simultaneously consistent and degenerate elliptic, hence accurate and robust in
applications. These properties are achieved by exploiting the arithmetic
structure of the discrete domain, assumed to be a two dimensional cartesian
grid. The construction of our scheme is simple, but its analysis relies on
original tools seldom encountered in numerical analysis, such as the geometry
of two dimensional lattices, and an arithmetic structure called the
Stern-Brocot tree. Numerical experiments illustrate the method's efficiency
Greedy bisection generates optimally adapted triangulations
We study the properties of a simple greedy algorithm for the generation of
data-adapted anisotropic triangulations. Given a function f, the algorithm
produces nested triangulations and corresponding piecewise polynomial
approximations of f.
The refinement procedure picks the triangle which maximizes the local Lp
approximation error, and bisect it in a direction which is chosen so to
minimize this error at the next step. We study the approximation error in the
Lp norm when the algorithm is applied to C2 functions with piecewise linear
approximations.
We prove that as the algorithm progresses, the triangles tend to adopt an
optimal aspect ratio which is dictated by the local hessian of f. For convex
functions, we also prove that the adaptive triangulations satisfy a convergence
bound which is known to be asymptotically optimal among all possible
triangulations.Comment: 24 page
Worst case and average case cardinality of strictly acute stencils for two dimensional anisotropic fast marching
We study a one dimensional approximation-like problem arising in the discretization of a class of Partial Differential Equations, providing worst case and average case complexity results. The analysis is based on the Stern-Brocot tree of rationals, and on a non-Euclidean notion of angles. The presented results generalize and improve on earlier work
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