42 research outputs found
FAST SOLUTION METHODS FOR CONVEX QUADRATIC OPTIMIZATION OF FRACTIONAL DIFFERENTIAL EQUATIONS
In this paper, we present numerical methods suitable for solving convex
quadratic Fractional Differential Equation (FDE) constrained optimization
problems, with box constraints on the state and/or control variables. We
develop an Alternating Direction Method of Multipliers (ADMM) framework, which
uses preconditioned Krylov subspace solvers for the resulting sub-problems. The
latter allows us to tackle a range of Partial Differential Equation (PDE)
optimization problems with box constraints, posed on space-time domains, that
were previously out of the reach of state-of-the-art preconditioners. In
particular, by making use of the powerful Generalized Locally Toeplitz (GLT)
sequences theory, we show that any existing GLT structure present in the
problem matrices is preserved by ADMM, and we propose some preconditioning
methodologies that could be used within the solver, to demonstrate the
generality of the approach. Focusing on convex quadratic programs with
time-dependent 2-dimensional FDE constraints, we derive multilevel circulant
preconditioners, which may be embedded within Krylov subspace methods, for
solving the ADMM sub-problems. Discretized versions of FDEs involve large dense
linear systems. In order to overcome this difficulty, we design a recursive
linear algebra, which is based on the Fast Fourier Transform (FFT). We manage
to keep the storage requirements linear, with respect to the grid size ,
while ensuring an order computational complexity per iteration of
the Krylov solver. We implement the proposed method, and demonstrate its
scalability, generality, and efficiency, through a series of experiments over
different setups of the FDE optimization problem
Spectral analysis for preconditioning of multi-dimensional Riesz fractional diffusion equations
In this paper, we analyze the spectra of the preconditioned matrices arising
from discretized multi-dimensional Riesz spatial fractional diffusion
equations. The finite difference method is employed to approximate the
multi-dimensional Riesz fractional derivatives, which will generate symmetric
positive definite ill-conditioned multi-level Toeplitz matrices. The
preconditioned conjugate gradient method with a preconditioner based on the
sine transform is employed to solve the resulting linear system. Theoretically,
we prove that the spectra of the preconditioned matrices are uniformly bounded
in the open interval (1/2,3/2) and thus the preconditioned conjugate gradient
method converges linearly. The proposed method can be extended to multi-level
Toeplitz matrices generated by functions with zeros of fractional order. Our
theoretical results fill in a vacancy in the literature. Numerical examples are
presented to demonstrate our new theoretical results in the literature and show
the convergence performance of the proposed preconditioner that is better than
other existing preconditioners
Diagonal and normal with Toeplitz-block splitting iteration method for space fractional coupled nonlinear Schr\"odinger equations with repulsive nonlinearities
By applying the linearly implicit conservative difference scheme proposed in
[D.-L. Wang, A.-G. Xiao, W. Yang. J. Comput. Phys. 2014;272:670-681], the
system of repulsive space fractional coupled nonlinear Schr\"odinger equations
leads to a sequence of linear systems with complex symmetric and
Toeplitz-plus-diagonal structure. In this paper, we propose the diagonal and
normal with Toeplitz-block splitting iteration method to solve the above linear
systems. The new iteration method is proved to converge unconditionally, and
the optimal iteration parameter is deducted. Naturally, this new iteration
method leads to a diagonal and normal with circulant-block preconditioner which
can be executed efficiently by fast algorithms. In theory, we provide sharp
bounds for the eigenvalues of the discrete fractional Laplacian and its
circulant approximation, and further analysis indicates that the spectral
distribution of the preconditioned system matrix is tight. Numerical
experiments show that the new preconditioner can significantly improve the
computational efficiency of the Krylov subspace iteration methods. Moreover,
the corresponding preconditioned GMRES method shows space mesh size independent
and almost fractional order parameter insensitive convergence behaviors
Preconditioned fast solvers for large linear systems with specific sparse and/or Toeplitz-like structures and applications
In this thesis, the design of the preconditioners we propose starts from applications instead of treating the problem in a completely general way. The reason is that not all types of linear systems can be addressed with the same tools. In this sense, the techniques for designing efficient iterative solvers depends mostly on properties inherited from the continuous problem, that has originated the discretized sequence of matrices. Classical examples are locality, isotropy in the PDE context, whose discrete counterparts are sparsity and matrices constant along the diagonals, respectively.
Therefore, it is often important to take into account the properties of the originating continuous model for obtaining better performances and for providing an accurate convergence analysis. We consider linear systems that arise in the solution of both linear and nonlinear partial differential equation of both integer and fractional type. For the latter case, an introduction to both the theory and the numerical treatment is given.
All the algorithms and the strategies presented in this thesis are developed having in mind their parallel implementation. In particular, we consider the processor-co-processor framework, in which the main part of the computation is performed on a Graphics Processing Unit (GPU) accelerator.
In Part I we introduce our proposal for sparse approximate inverse preconditioners for either the solution of time-dependent Partial Differential Equations (PDEs), Chapter 3, and Fractional Differential Equations (FDEs), containing both classical and fractional terms, Chapter 5. More precisely, we propose a new technique for updating preconditioners for dealing with sequences of linear systems for PDEs and FDEs, that can be used also to compute matrix functions of large matrices via quadrature formula in Chapter 4 and for optimal control of FDEs in Chapter 6. At last, in Part II, we consider structured preconditioners for quasi-Toeplitz systems. The focus is towards the numerical treatment of discretized convection-diffusion equations in Chapter 7 and on the solution of FDEs with linear multistep formula in boundary value form in Chapter 8
Preconditioned fast solvers for large linear systems with specific sparse and/or Toeplitz-like structures and applications
In this thesis, the design of the preconditioners we propose starts from applications instead of treating the problem in a completely general way. The reason is that not all types of linear systems can be addressed with the same tools. In this sense, the techniques for designing efficient iterative solvers depends mostly on properties inherited from the continuous problem, that has originated the discretized sequence of matrices. Classical examples are locality, isotropy in the PDE context, whose discrete counterparts are sparsity and matrices constant along the diagonals, respectively.
Therefore, it is often important to take into account the properties of the originating continuous model for obtaining better performances and for providing an accurate convergence analysis. We consider linear systems that arise in the solution of both linear and nonlinear partial differential equation of both integer and fractional type. For the latter case, an introduction to both the theory and the numerical treatment is given.
All the algorithms and the strategies presented in this thesis are developed having in mind their parallel implementation. In particular, we consider the processor-co-processor framework, in which the main part of the computation is performed on a Graphics Processing Unit (GPU) accelerator.
In Part I we introduce our proposal for sparse approximate inverse preconditioners for either the solution of time-dependent Partial Differential Equations (PDEs), Chapter 3, and Fractional Differential Equations (FDEs), containing both classical and fractional terms, Chapter 5. More precisely, we propose a new technique for updating preconditioners for dealing with sequences of linear systems for PDEs and FDEs, that can be used also to compute matrix functions of large matrices via quadrature formula in Chapter 4 and for optimal control of FDEs in Chapter 6. At last, in Part II, we consider structured preconditioners for quasi-Toeplitz systems. The focus is towards the numerical treatment of discretized convection-diffusion equations in Chapter 7 and on the solution of FDEs with linear multistep formula in boundary value form in Chapter 8
Spectral features of matrix-sequences, GLT, symbol, and application in preconditioning Krylov methods, image deblurring, and multigrid algorithms.
The final purpose of any scientific discipline can be regarded as the solution of real-world problems. With this aim, a mathematical modeling of the considered phenomenon is often compulsory. Closed-form solutions of the arising functional equations are usually not available and numerical discretization techniques are required. In this setting, the discretization of an infinite-dimensional linear equation via some linear approximation method, leads to a sequence of linear systems of increasing dimension whose coefficient matrices could inherit a structure from the continuous problem. For instance, the numerical approximation by local methods of constant or nonconstant coefficients systems of Partial Differential Equations (PDEs) over multidimensional domains, gives rise to multilevel block Toeplitz or to Generalized Locally Toeplitz (GLT) sequences, respectively. In the context of structured matrices, the convergence properties of iterative methods, like multigrid or preconditioned Krylov techniques, are strictly related to the notion of symbol, a function whose role relies in describing the asymptotical distribution of the spectrum.
This thesis can be seen as a byproduct of the combined use of powerful tools like symbol, spectral distribution, and GLT, when dealing with the numerical solution of structured linear systems. We approach such an issue both from a theoretical and practical viewpoint. On the one hand, we enlarge some known spectral distribution tools by proving the eigenvalue distribution of matrix-sequences obtained as combination of some algebraic operations on multilevel block Toeplitz matrices. On the other hand, we take advantage of the obtained results for designing efficient preconditioning techniques. Moreover, we focus on the numerical solution of structured linear systems coming from the following applications: image deblurring, fractional diffusion equations, and coupled PDEs. A spectral analysis of the arising structured sequences allows us either to study the convergence and predict the behavior of preconditioned Krylov and multigrid methods applied to the coefficient matrices, or to design effective preconditioners and multigrid solvers for the associated linear systems
Preconditioned iterative methods for optimal control problems with time-dependent PDEs as constraints
In this work, we study fast and robust solvers for optimal control problems with
Partial Differential Equations (PDEs) as constraints. Speci cally, we devise preconditioned
iterative methods for time-dependent PDE-constrained optimization
problems, usually when a higher-order discretization method in time is employed
as opposed to most previous solvers. We also consider the control of stationary
problems arising in
uid dynamics, as well as that of unsteady Fractional Differential
Equations (FDEs). The preconditioners we derive are employed within an
appropriate Krylov subspace method.
The fi rst key contribution of this thesis involves the study of fast and robust
preconditioned iterative solution strategies for the all-at-once solution of optimal
control problems with time-dependent PDEs as constraints, when a higher-order
discretization method in time is employed. In fact, as opposed to most work in
preconditioning this class of problems, where a ( first-order accurate) backward
Euler method is used for the discretization of the time derivative, we employ a
(second-order accurate) Crank-Nicolson method in time. By applying a carefully
tailored invertible transformation, we symmetrize the system obtained, and
then derive a preconditioner for the resulting matrix. We prove optimality of the
preconditioner through bounds on the eigenvalues, and test our solver against a
widely-used preconditioner for the linear system arising from a backward Euler
discretization. These theoretical and numerical results demonstrate the effectiveness
and robustness of our solver with respect to mesh-sizes and regularization
parameter. Then, the optimal preconditioner so derived is generalized from the
heat control problem to time-dependent convection{diffusion control with Crank-
Nicolson discretization in time. Again, we prove optimality of the approximations
of the main blocks of the preconditioner through bounds on the eigenvalues, and,
through a range of numerical experiments, show the effectiveness and robustness
of our approach with respect to all the parameters involved in the problem.
For the next substantial contribution of this work, we focus our attention on
the control of problems arising in
fluid dynamics, speci fically, the Stokes and the
Navier-Stokes equations. We fi rstly derive fast and effective preconditioned iterative
methods for the stationary and time-dependent Stokes control problems, then
generalize those methods to the case of the corresponding Navier-Stokes control
problems when employing an Oseen approximation to the non-linear term. The
key ingredients of the solvers are a saddle-point type approximation for the linear
systems, an inner iteration for the (1,1)-block accelerated by a preconditioner for
convection-diffusion control problems, and an approximation to the Schur complement
based on a potent commutator argument applied to an appropriate block
matrix. Through a range of numerical experiments, we show the effectiveness of
our approximations, and observe their considerable parameter-robustness.
The fi nal chapter of this work is devoted to the derivation of efficient and robust
solvers for convex quadratic FDE-constrained optimization problems, with
box constraints on the state and/or control variables. By employing an Alternating
Direction Method of Multipliers for solving the non-linear problem, one can
separate the equality from the inequality constraints, solving the equality constraints
and then updating the current approximation of the solutions. In order
to solve the equality constraints, a preconditioner based on multilevel circulant
matrices is derived, and then employed within an appropriate preconditioned
Krylov subspace method. Numerical results show the e ciency and scalability of
the strategy, with the cost of the overall process being proportional to N log N,
where N is the dimension of the problem under examination. Moreover, the strategy
presented allows the storage of a highly dense system, due to the memory
required being proportional to N
Spectral features of matrix-sequences, GLT, symbol, and application in preconditioning Krylov methods, image deblurring, and multigrid algorithms.
The final purpose of any scientific discipline can be regarded as the solution of real-world problems. With this aim, a mathematical modeling of the considered phenomenon is often compulsory. Closed-form solutions of the arising functional equations are usually not available and numerical discretization techniques are required. In this setting, the discretization of an infinite-dimensional linear equation via some linear approximation method, leads to a sequence of linear systems of increasing dimension whose coefficient matrices could inherit a structure from the continuous problem. For instance, the numerical approximation by local methods of constant or nonconstant coefficients systems of Partial Differential Equations (PDEs) over multidimensional domains, gives rise to multilevel block Toeplitz or to Generalized Locally Toeplitz (GLT) sequences, respectively. In the context of structured matrices, the convergence properties of iterative methods, like multigrid or preconditioned Krylov techniques, are strictly related to the notion of symbol, a function whose role relies in describing the asymptotical distribution of the spectrum.
This thesis can be seen as a byproduct of the combined use of powerful tools like symbol, spectral distribution, and GLT, when dealing with the numerical solution of structured linear systems. We approach such an issue both from a theoretical and practical viewpoint. On the one hand, we enlarge some known spectral distribution tools by proving the eigenvalue distribution of matrix-sequences obtained as combination of some algebraic operations on multilevel block Toeplitz matrices. On the other hand, we take advantage of the obtained results for designing efficient preconditioning techniques. Moreover, we focus on the numerical solution of structured linear systems coming from the following applications: image deblurring, fractional diffusion equations, and coupled PDEs. A spectral analysis of the arising structured sequences allows us either to study the convergence and predict the behavior of preconditioned Krylov and multigrid methods applied to the coefficient matrices, or to design effective preconditioners and multigrid solvers for the associated linear systems