Questions tagged [krylov-method]
Referring to Krylov Subspaces and the methods of solutions to linear systems of equations which exploit these spaces.
13
questions
27
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Why is my iterative linear solver not converging?
What can go wrong when using preconditoned Krylov methods from KSP (PETSc's linear solver package) to solve a sparse linear system such as those obtained by discretizing and linearizing partial ...
24
votes
3answers
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What is the principle behind the convergence of Krylov subspace methods for solving linear systems of equations?
As I understand it, there are two major categories of iterative methods for solving linear systems of equations:
Stationary Methods (Jacobi, Gauss-Seidel, SOR, Multigrid)
Krylov Subspace methods (...
6
votes
3answers
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role of initial guess for iterative linear solver
Suppose we use a preconditioned iterative solver for a linear system. If the initial state for the solver can be chosen very close to the exact solution - does this reduce requirements for the ...
15
votes
1answer
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Can a Krylov subspace method be used as a smoother for multigrid?
As far as I am aware, multigrid solvers use iterative smoothers such as Jacobi, Gauss-Seidel, and SOR to dampen the error at various frequencies. Could a Krylov subspace method (like conjugate ...
12
votes
2answers
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Which preconditioners (and solver) in PETSc for indefinite symmetric systems should I use?
My system is a symmetric FE problem with lagrange multipliers (e.g. incompressible Stokes' flow):
\begin{pmatrix}A & B^T \\ B & C\end{pmatrix}
where $C = 0$ is the typical case (I have even ...
13
votes
1answer
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Why is pinning a point to remove a null space bad?
A Poisson equation with all Neumann boundary conditions has a single constant dimensional null space. When solving via a Krylov method, the null space can be removed either by subtracting the mean of ...
8
votes
2answers
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Why does conjugate gradient work with this nonsymmetric preconditioner?
In this previous thread the following multiplicative way to combine symmetric preconditioners $P_1$ and $P_2$ for the symmetric system $Ax=b$ was suggested:
\begin{align}
P_\text{combo}^{-1} :=& ...
6
votes
2answers
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Krylov subspace iterative methods in floating point arithmetic
Is there any work that considers Krylov subspace iterative methods in floating point arithmetic? I'm especially interested in how rounding errors influence the convergence and the accuracy of the ...
14
votes
1answer
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How do low rank modifications affect Krylov method convergence?
Say I have a linear system $A x = b$, which converges quickly using a suitable Krylov method (such as CG or GMRES) for all $b$. If $B$ is a matrix with low rank $r$, will the same Krylov method on ...
15
votes
2answers
449 views
Is there any way to do “double preconditioning”
Question:
Suppose that you have two different (factored) preconditioners for a symmetric positive definite matrix $A$:
$$A \approx B^TB$$
and
$$A \approx C^TC,$$
where the inverses of the factors $B, ...
3
votes
1answer
406 views
How to implement flexible gmres in matlab?
About the flexible GMRES (fgmres), we know that it is a variant of right preconditioned gmres. And the robust command gmres in matlab as follows:
...
5
votes
3answers
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PRIMA gives an unstable result?
I am working with Modified Nodal Admittance representation of circuits. I am doing Model Order Reduction using PRIMA on MATLAB. I am considering these circuits as Descriptor State-Space systems.
I ...
4
votes
1answer
201 views
Choosing preconditioner for unsymmetric pressure-velocity coupled system
I'm working with pressure-velocity coupled systems. It means that instead of solving 4 different linear systems in segregated approach (1 for pressure and 3 for Ux, Uy, Uz), we can solve only one ...
