# Computational Science Stack Exchange 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

### 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 ...

floating-point

### 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

### 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

### 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

### 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

### 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 \...

python convex-optimization cvxpy
 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

### 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

### 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

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}^\...

optimization
 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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