# All Questions

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### Stationary solution converge but time dependent doesn't

I've coupled a COMSOL model for fluid dynamics with a very simple pde that model the transport of humidity in air. When I solve it for the stationary case, the solution converge easily, but when I ...
615 views

### Efficient computation of Markov chain transition probability matrix

Consider a continuous Markov chain $X=(X_t)$ on a finite state space and let $Q$ be the (given) transition rate matrix. This matrix is very sparse, with non-zero values on 3 diagonals only (so from ...
711 views

### How to do local FFT on huge 3D vector data cell mesh and visualize it spatially?

Simulation type: I'm running a simulation with the OOMMF micromagnetics package http://math.nist.gov/oommf/ where are magnet is represented by a mesh of 3 million cells, it gets excited by a ...
191 views

### Under which conditions is better OMP_NUM_THREADS = 4 or 2 for multithread two-core CPU?

I am running my Fortran 95 numerical codes, whose structure is most times easily parallelized by little more than simply adding the corresponding OpenMP instruction before some critical DO sentences, ...
85 views

### How to create synthetic data from known weights

I'm doing some machine learning where I have lots of data and through optimization I'm trying to learn the weights for the model. I'd like to check that my learning actually works correctly. For that ...
121 views

### explicitly forming coarse matrices with polynomial smoothing AMG

I've been reading about the algebraic multigrid algorithm and came across polynomial smoothers in this paper. It's my understanding that usually the coarse-level matrices $A_H = I_h^HA_hI_H^h$ are ...
6k views

### Fortran: Best way to time sections of your code?

Sometimes while optimizing code it is required to time certain portions of the code, I have been using the following for years but was wondering if there is a simpler/better way to do it? ...
248 views

### Solving a linear system with matrix arguments

We're all familiar with the many computational methods to solve the standard linear system $$Ax=b.$$However, I'm curious if there are any "standard" computational methods for solving a more ...
7k views

### How to approximate the condition number of a large matrix?

How do I approximate the condition number of a large matrix $G$, if $G$ is a combination of Fourier transforms $F$ (non-uniform or uniform), finite differences $R$, and diagonal matrices $S$? The ...
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### Solver for a MIQP with an indefinite coefficient matrix

Do CPLEX or Gurobi handle MIQPs with indefinite coefficient matrices? The problem I am dealing with has quadratic terms in which one variable is binary and the other variable is continuous. The ...
223 views

### Are there any specialized methods available for solving structurally symmetric sparse linear systems?

When solving $Ax=b$, prior knowledge about $A$'s structure can help in designing an efficient solver which exploits this information (e.g conjugate gradient method is to be used when $A$ is ...
1k views

### Solver error in SciPy/LSODA with a very specific parameter set

I'm implementing a very simple Susceptible-Infected-Recovered model with a steady population for an idle side project - normally a pretty trivial task. But I'm running into solver errors using either ...
166 views

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### Location of Unknowns in Unstructured Mesh

I am currently learning a code which utilizes Scharfetter-Gummel discretization for unsteady drift-diffusion equations. For this scheme, a 2D unstructured triangular mesh is used, with the unknowns ...
92 views

### mapping data with a spike to a heat map

I have the following dataset that I need to display on the heat map: [30, 15, 66, 7, 9999, 78, 42, 132] So if I map the values to the color scale using a linear function I only see the spike while ...