# All Questions

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### How can the choice of coarsening factor affect Multigrid's convergence?

The linear system $Ax=b$ is coming from the discretization of an elliptic PDE. Multigrid method is used in order to solve it. Suppose $c_0$ is the coarsening factor on level 0 and $c_m$ the coarsening ...
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### How to refine the tetrahedron if exist two longest length edge?

In some algorithms to refine tetrahedron, we need to calculate the longest edge. background If exist a tetrahedron with node ABCD, it has edges ...
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### Global to local transformation matrix in 2D frame structures

In section 3.2 of this paper , where 2D planar frame structures are being analyzed, the authors mentioned a transformation matrix to be used in extracting the element displacement vector from the ...
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### Guidelines for publishing data from a stochastic simulation

So, my question is if one should ideally keep a record of all seeds that are used when publishing numerical work that involves one or more random number generators (e.g. a stochastic simulation), and ...
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### 3d surface image for given datasets

I have some data that looks like as follows x-cor y-cor z-cor 0.02 0.0251 0.02 0.01 0.0257 0.02 0.014 0.02 0.03 0.023 0.003 0.05 0.013 0.002 0.77 .................... .................... ...
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### Is there a simple way to avoid carbuncles for FD WENO methods?

I have implemented finite-difference WENO scheme for Euler equations (with some variants - WENO-JS, WENO-Z, WENO-M, different flux splitting). It works well, but have problem with so-called carbuncles ...
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### Clever ways to update LU factorization for ridge regression [duplicate]

Ridge regression can be posed as minimizing the following objective function (over $x$): $$\frac{1}{2} \lVert Ax - b \lVert_2^2 ~+ \frac{\lambda}{2} \lVert x \lVert_2^2$$ Which has a closed form ...
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### “WY” representation of QR factorization — implementations?

I have a matrix $A \in \mathbb{R}^{m \times n}$ where $m \gg n$ and I want to compute the full QR decomposition $A = QR$. Where $Q$ is an orthogonal $m \times m$ matrix. Bishof & Van Loan (1987) ...
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### Grid Independence Study

Is the change in time step necessary for the grid independent study? As the CFL is based on the relation between dt and dx. In mesh independent study, only change should be mesh i.e, dx isn't it so?
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### Help understanding Brent's root finding method

Help me understand a part of Brent's root finding algorithm. In a typical iteration we have samples (a,fa), (b,fb), (c,fc) all real with (a<b<c) or (c<b<a) . Also, in the case I am ...
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### Split-step Fourier method applied on Schrodinger equation

I'm trying to solve a Schrodinger equation of the form $i\frac{\partial}{\partial t}\psi=-\frac{\partial^2}{\partial x^2}\psi + (V(x)+\alpha|\psi|^2)\psi$ using the split-step Fourier method ...
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### Task Spooler – Executing an Executable within an Executable

I am currently using Task Spooler, which is a job scheduler I have installed on my iMac. I have encountered a problem in which I cannot execute a job properly. Using a bash script, this is how I run ...
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### Multigrid Reduction In Time Convergence

I am trying to solve a 2D dynamic linear elasticity model parallel in time using Xbraid. The spatial domain is [0,1]x[0,1] and time domain [0,1]. For time integration I am using a backward Euler ...
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### Error in Monte Carlo integration

I am looking for a concise description to help me understand the error for Monte Carlo Integration using Uniform Sampling and Importance Sampling For Importance Sampling I have that the error is just ...
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### discretizing advection equation with variable wave speed + stability

I currently have a code that solves $u_t+ cu_x=0$ with periodic boundary conditions, and constant $c$ (using an upwind method). I'm wondering how I would alter this code to solve something of the form ...
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### Clarification regarding 3D FMM translation operators

I am implementing an adaptive 3D FMM with the "basic" $O(p^4)$ translation operators. I am looking for clarification on the multipole-to-multipole (M2M) translation operator. I will explain my ...
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### Determining the voxels between two boundary surfaces

Update (Solution) -17 October 2020- I finally did it! You can check the code on my GitLab repo. The function is to do this is the domain_extract which identifies ...