71,926 research outputs found
An inexact Newton-Krylov algorithm for constrained diffeomorphic image registration
We propose numerical algorithms for solving large deformation diffeomorphic
image registration problems. We formulate the nonrigid image registration
problem as a problem of optimal control. This leads to an infinite-dimensional
partial differential equation (PDE) constrained optimization problem.
The PDE constraint consists, in its simplest form, of a hyperbolic transport
equation for the evolution of the image intensity. The control variable is the
velocity field. Tikhonov regularization on the control ensures well-posedness.
We consider standard smoothness regularization based on - or
-seminorms. We augment this regularization scheme with a constraint on the
divergence of the velocity field rendering the deformation incompressible and
thus ensuring that the determinant of the deformation gradient is equal to one,
up to the numerical error.
We use a Fourier pseudospectral discretization in space and a Chebyshev
pseudospectral discretization in time. We use a preconditioned, globalized,
matrix-free, inexact Newton-Krylov method for numerical optimization. A
parameter continuation is designed to estimate an optimal regularization
parameter. Regularity is ensured by controlling the geometric properties of the
deformation field. Overall, we arrive at a black-box solver. We study spectral
properties of the Hessian, grid convergence, numerical accuracy, computational
efficiency, and deformation regularity of our scheme. We compare the designed
Newton-Krylov methods with a globalized preconditioned gradient descent. We
study the influence of a varying number of unknowns in time.
The reported results demonstrate excellent numerical accuracy, guaranteed
local deformation regularity, and computational efficiency with an optional
control on local mass conservation. The Newton-Krylov methods clearly
outperform the Picard method if high accuracy of the inversion is required.Comment: 32 pages; 10 figures; 9 table
Efficient Construction, Update and Downdate Of The Coefficients Of Interpolants Based On Polynomials Satisfying A Three-Term Recurrence Relation
In this paper, we consider methods to compute the coefficients of
interpolants relative to a basis of polynomials satisfying a three-term
recurrence relation. Two new algorithms are presented: the first constructs the
coefficients of the interpolation incrementally and can be used to update the
coefficients whenever a nodes is added to or removed from the interpolation.
The second algorithm, which constructs the interpolation coefficients by
decomposing the Vandermonde-like matrix iteratively, can not be used to update
or downdate an interpolation, yet is more numerically stable than the first
algorithm and is more efficient when the coefficients of multiple
interpolations are to be computed over the same set of nodes.Comment: 18 pages, submitted to the Journal of Scientific Computing
Cost-Benefit Analysis of the Large Hadron Collider to 2025 and beyond
Social cost-benefit analysis (CBA) of projects has been successfully applied
in different fields such as transport, energy, health, education, and
environment, including climate change. It is often argued that it is impossible
to extend the CBA approach to the evaluation of the social impact of research
infrastructures, because the final benefit to society of scientific discovery
is generally unpredictable. Here, we propose a quantitative approach to this
problem, we use it to design an empirically testable CBA model, and we apply it
to the the Large Hadron Collider (LHC), the highest-energy accelerator in the
world, currently operating at CERN. We show that the evaluation of benefits can
be made quantitative by determining their value to users (scientists,
early-stage researchers, firms, visitors) and non-users (the general public).
Four classes of contributions to users are identified: knowledge output, human
capital development, technological spillovers, and cultural effects. Benefits
for non-users can be estimated, in analogy to public goods with no practical
use (such as environment preservation), using willingness to pay. We determine
the probability distribution of cost and benefits for the LHC since 1993 until
planned decommissioning in 2025, and we find there is a 92% probability that
benefits exceed its costs, with an expected net present value of about 3
billion euro, not including the unpredictable economic value of discovery of
any new physics. We argue that the evaluation approach proposed here can be
replicated for any large-scale research infrastructure, thus helping the
decision-making on competing projects, with a socio-economic appraisal
complementary to other evaluation criteria.Comment: 17 pages, 7 figure
Energy conserving time integration scheme for geometrically exact beam
An energy conserving finite-element formulation for the dynamic analysis of geometrically non-linear beam-like structures undergoing large overall motions has been developed. The formulation uses classical displacement-based planar beam finite elements described in an inertial frame. It takes into account finite axial, bending and shear strains. A theoretically consistent approach is used to derive a novel and simple energy conserving scheme, using the unconventional incremental strain update rather than the standard strong form. Numerical examples demonstrate perfect energy and momenta conservation, stability and robustness of the scheme, and good convergence properties in terms of both the Newton-Raphson method and time step size. (c) 2006 Elsevier B.V. All rights reserved
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