Community Digest

Top new questions this week:

Exponent log to compute reciprocal power?

A MATLAB library seems to overcomplicate a computation: exp( (log(a) - log(b))/b ) which is mathematically equivalent (assuming real & positive ...

matlab python algorithms stability numerical-limitations  
asked by OverLordGoldDragon 5 votes
answered by njuffa 4 votes

How frequently scientific code uses comparisons NaN == NaN?

How frequently scientific code uses comparisons NaN == NaN? Reason of asking: from time to time compilers / software floating-point library implementations have ...

asked by pmor 3 votes
answered by MPIchael 3 votes

Algorithm to merge two polygons (using connectivities)?

I am struggling with implementing an algorithm that does one simple thing: Consider two polygons (one can just draw any two polygons and number their vertices), whose connectivities in a node list are:...

algorithms computational-geometry geometry vector  
asked by lucmobz 3 votes
answered by Wolfgang Bangerth 2 votes

Nonlinear root solving libraries which accept a Jacobian in band-storage

I'm in search for a library for solving large systems of non-linear equations, similar to MINPACK, but unlike MINPACK, can accept a Jacobian in band-storage. My Jacobian is sometimes not invertible, ...

optimization nonlinear-equations sparse-matrix rootfinding  
asked by nicholaswogan 3 votes
answered by IPribec 2 votes

Quick way to find a common basis of eigenvectors between 2 matrices : valid or not?

Following the advise of @Federico Polonion a previous post, one suggested, to find a basis of common eigen vectors between 2 matrices, to simply do : Generate 2 ...

matrix eigenvalues matrix-factorization basis-set randomized-algorithms  
asked by youpilat13 3 votes
answered by Federico Poloni 3 votes

How to Invert a Poorly Conditioned Matrix

In my research, I need to invert a Fisher matrix in order to get a covariance matrix for me to do parameter estimation. Unfortunately, the values of Fisher matrix vary by many orders of magnitude, and ...

matrix numpy condition-number inverse  
asked by Brian.C.Seymour 2 votes
answered by Federico Poloni 3 votes

Why is CVXPY throwing a DCP error with cp.sqrt but no error with cp.norm

I am trying to use CVXPY to optimise signal-to-noise-plus interference ratio (SINR) for a visible light communication (VLC) system. I have one of my SINR constraints stated as \begin{equation} \...

python convex-optimization cvxpy  
asked by Supremum 2 votes
answered by Mark L. Stone 1 vote

Greatest hits from previous weeks:

What is the difference between implicit FEM and explicit FEM?

What is the difference between explicit FEM and implicit FEM exactly? According to the post here, it seems that the only difference is whether implicit or explicit time integration is used. As I ...

finite-element implicit-methods explicit-methods  
asked by Fei Zhu 10 votes
answered by Stefano M 7 votes

Visually appealing ways to plot singular vector fields with matplotlib or other foss tools

What is the best way to get a visual appealing plot of a singular vector field (if you want to visualize also the field strength). As an example I am playing with the electric fields of two point ...

python visualization  
asked by Julia 10 votes
answered by Henri Menke 7 votes

In FEM, why is the stiffness matrix positive definite?

In FEM classes, it's usually taken for granted that the stiffness matrix is positive definite, but I just can't understand why. Could anyone give some explanation? For instance, we can consider the ...

finite-element matrix stiffness  
asked by user123 12 votes

How to solve block tridiagonal matrix using Thomas algorithm

Thomas algorithm can be used to solve a tridiagonal matrix: $$ \begin{bmatrix} {b_ 1} & {c_ 1} & { } & { } & { 0 } \\ {a_ 2} & {b_ 2} & {c_ 2} & { } & { }...

algorithms linear-solver sparse-matrix  
asked by xslittlegrass 6 votes
answered by Shainath 1 vote

Concave polygon 'hull' finding

I implemented an algorithm to find the alpha shape of a set of points. The alpha shape is a concave hull for a set of points, whose shape depends on a parameter alpha deciding which points make up the ...

algorithms computational-geometry graph-theory convex-hull  
asked by kiriloff 6 votes

Python vs FORTRAN

Which one is better: FORTRAN or Python? And I guess that in both cases you need Gnuplot, am I right? I'm working on a Windows machine at the moment. I'd like to use it to get numerical solutions for ...

python fortran  
asked by Nick 19 votes
answered by Geoff Oxberry 32 votes

How should boundary conditions be applied when using finite-volume method?

Following from my previous question I am trying to apply boundary conditions to this non-uniform finite volume mesh, I would like to apply a Robin type boundary condition to the l.h.s. of the domain (...

boundary-conditions finite-volume numerical-analysis  
asked by boyfarrell 16 votes

Can you answer these questions?

Computing matrices for Model Predictive Control

In model predictive control, an optimization problem is solved at every time instant and it is very common to write down the matrices in a compact form. Without going into the details of the ...

matlab matrix matrix-equations  
asked by cholo14 1 vote

Minimize MAE for sum of powers

The model is $$y_i=\beta\big(x_{i1}^\alpha+x_{i2}^\alpha+...+x_{im_i}^\alpha\big)+\epsilon_i\textrm{ for }i=1, 2, ..., n$$ I want to minimize the MAE, i.e. $$\Sigma_{i=1}^n{\big|y_i-\beta\big(x_{i1}^\...

asked by Yanqi Huang 1 vote

Model and solve nutrition optimization problem: how to?

How to solve/assess the following problem? Given: $N$ ingredients like apples, bread etc. Mass of an ingredient $j$ is in this simplified model a sum of macro nutrients carbohydrates, proteins and ...

constrained-optimization linear-programming  
asked by Open Food Broker 1 vote
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