# All Questions

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### Will future Operational Systems adopt cyclic trees for directory organisation?

The directory structure in current file systems consists of directed acyclic trees of folders were subordinate directories can belong to only one parent directory. This poses radical limitations to ...
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### Octave is telling me I have undefined constants, confused why code isn't working

his should be a really simple little piece of code and yet somehow it isn't running in octave, any help at all will be greatly appreciated! Here is my code: t=pi/2; i=sqrt(-1); ...
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### Fortran95 for IOS/android [on hold]

i am a engineer working for offshore oil/gas field, and i am good with fortran 95. i have written few programs in f95 specific for my work, which i feel would be great if they worked on my ...
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### Big data analysis project ideas? [on hold]

I recently renovated a desktop that had been in storage for a while, and I'd like to put it to use analyzing some big data sets or testing some algorithms or something. Does anyone have any ...
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### Does there exist a Fourier transform algorithm for perturbed data?

Assuming I have a length-$n$ real vector $x$ and have already computed its Fourier transform $\hat x$ (in time $O(n\log n)$), I would like to compute the Fourier transform of $y = x + \delta x$, where ...
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### Convert an integer to binary using base 2

Given $A$, an array, $i$, the array index, $A[i] \in \{1,0\}$, $M=\sum_i(A[i] * b^i)$, where $b= 2$ I need a function, which given an integer $N$ returns the array representing such number. ...
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### How can I generate 3D unstructured mesh on MATLAB

I have been looking to do a 3D mesh for a shallow tunnel FE analysis. The output of mesh node orientation along the element should be counter clock wise direction. How and which material helps me best ...
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### FEniCS: boundary conditions with explicit timestepping

I'm trying to implement RK4 scheme in elastic wave equation - stress-velocity formulation. I've extracted assembled matrix and went to the time loop, but the initial condition does not propagate at ...
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### Understanding the conditions for which ADMM can be applied

While reading Boyd's paper on ADMM I encountered an issue. Consider the following problem: Problem. Minimize $f(u) + g(v)$ subject to $Au + Bv = c$, where $f$ and $g$ are closed, proper, convex and ...
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### Symplectic integration of PDE

I consider ordinary wave equation $$u_{tt} - u_{xx} = 0$$ with initial conditions $$u(x, 0) = \exp (-2 x^2) \\ u_t(x, 0) = 0$$ To solve this problem I approximate $u_{xx}$ with 4-th order ...
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### Indexing Nested Loops in C

I am having trouble indexing correctly the below statement in C inside a function and then returning it as a pointer. The returning part should not be confusing - hopefully - however the indexing is a ...
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### Angle of rotation at a point in a deformed triangle

I have a 2D triangle which deforms with each vertex moving by some small ($\sin(x) \approx \tan(x) \approx x$) displacement vector. The displacement of any point in the triangle is linearly ...
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### Solving compressible inviscid Euler equations with shockwaves in polar coordinates

For the past several weeks I was attempting to adapt Lax-Wendroff or some similar scheme for polar coordinates. The process was complicated due to me being unable to find step-by-step derivations of ...
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### Adding a new potential term to DL_POLY Classic

I am using DL_POLY Classic to simulate CO2 behavior in upper mantle conditions. I am trying to use a recently new forcefield for this system. In this FF, the potential energy between two atoms in the ...
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### Projection on Stiefel manifold after integration step

A few days ago, I asked how constraints like $A^T A = I$ can be implemented if one wishes to integrate differential equations of the form $\dot{A}=f(A,t)$. Kirill was so kind to point out that a ...
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### efficient MPI collective non-blocking communication

I am facing a problem with MPI (Fortran). I have a really big matrix at each node and they differ at different nodes. At some point of my calculations, each node needs the matrix from all other nodes, ...
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### Fitting a rectangle to a point set

I have an ordered list of (2d-)points that are forming a (not axis aligned) rectangle and I'd like to recover that rectangle. Approximations like a minimal enclosing rectangle can't be used so that ...
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### How to deal with transition elements in adaptive fem

It is necessary for me to solve a Poisson equation by adaptive finite element method with transition elements technique to get conforming mesh. For the first local refinement everything is OK, ...
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### Non-uniform finite difference Adaptive Mesh Refinement

Assuming that the crosses in the figure below are unknowns in a vertex-centered finite difference scheme in an Adaptive Mesh, how can I calculate the double derivate (Laplacian) at the Red x ? The Red ...
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### How to cluster words using Jensen-Shannon Divergence?

I'm working on a project that requires text analyzing. I'm currently trying to extract keywords from a single document. I referred to this thesis(Keyword Extraction from a Single Document using Word ...
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### Efficient Quadrature Methods for Indicator Functions?

I am looking to numerically solve many different integrals where the integrand is simply the indicator function for a region (i.e. 1 on the region, 0 outside. This is for measuring areas). The ...
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### pointwise vs. continuous observations in PDE inverse problem

I work on an inverse problem for my Ph.D. research, which for simplicity's sake we'll say is determining $\beta$ in $L(\beta)u \equiv -\nabla\cdot(k_0e^\beta\nabla u) = f$ from some observations ...
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### Integration of differential equation with orthogonality constraint

Lets say I have a system of differential equations which has the form $$\dot{C}_{\alpha,\beta,m} = f_{\alpha,\beta,m}(C_{\alpha,\beta,1},\ldots,C_{\alpha,\beta,N};t).$$ The $f$s are some functions of ...
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### How to find active region in treepartition Matlab object

I am trying to identify a non-linear plant using the identification toolbox of Matlab. In particular I would like to identify a non-linear arx model, where the nonlinearities are expressed by means ...
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### Proof of CFL condition for RKDG scheme

The cfl condition for linear advection equation $$u_t + a u_x = 0$$ using a DG method of degree $k$ polynomials, upwind flux and an RK scheme of $k+1$ stage/accuracy is stated to be ...
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### Solving coupled PDE in COMSOL [closed]

I have the system of equations \begin{align} &A \frac{\partial u_1}{\partial t} = 1 - u_1 B \frac{\partial u_2}{\partial y}\\ &\frac{\partial u_2}{\partial t} = \frac{\partial}{\partial ...
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### Entering an Electric field dependent equation in material contents in COMSOL?

I am trying to model a CNTFET in COMSOL. I am following the same steps that were used to model a MOSFET (provided in the semiconductor module manual), except for a few additional layers and materials. ...
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### Monte Carlo Metropolis method - trial step algorithm

I'm working on a Magnetization simulation and writing an algorithm using the metropolis method. I am using a change in energy and a Boltzmann distribution, but, my question is about the trial step. ...
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### Data analysis of a magnetic hysteresis loop

I am a physics undergrad and I have MOKE data for a magnetic material (thin permalloy film on a silicon substrate). Here is one of the hysteresis loops I obtained, plotted using Python: The form of ...
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### Choice of basis in FEM

Briefly, what are the different types of basis used in FEM and why is the nodal basis so popular and advantageous in the finite element context?