# All Questions

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### Higher-order numerical integration on a triangle/tetrahedron/simplex

Let $T$ be a triangle and let $f$ be a smooth function on $T$. We can use mid-point quadrature $\int f dx \approx |T|\cdot f(x_M)$, where $x_M$ is the middle-point of $T$. Can you provide me with (a ...
804 views

### Construction of $C^1$/$H^2$-conforming finite element basis for triangular or tetrahedral mesh

In the paper Hierarchical Conforming Finite Element Methods for the Biharmonic Equation, P. Oswald claimed Clough-Tocher type elements has $C^1$-continuity while being a cubic polynomial on each ...
2k views

### Where do the laws of quantum mechanics break down in simulations?

As someone who holds a BA in physics I was somewhat scandalized when I began working with molecular simulations. It was a bit of a shock to discover that even the most detailed and computationally ...
14k views

### scipy.optimize.fmin_bfgs: “Desired error not necessarily achieved due to precision loss”

I am getting the warning in the post subject when attempting to optimize a function in Python with the scipy.optimize.fmin_bfgs function. The complete output: Warning: Desired error not necessarily ...
186 views

### How to establish that an iterative method can be applied to large matrices whose size may reach 10^3?

I have an iterative method for computing the Moore-Penrose generalized inverse of matrices, that is $$X_{k+1} = ((I-\beta X_{k}A)^t) + X_{k}$$ with initial approximation: $$X_{0} = \beta AA^t$$ ...
1k views

### Why would a computational scientist need to implement their own version of std::complex?

Many of the better-known C++ libraries in computational science such as Eigen, Trilinos, and deal.II use the standard C++ template header library object, ...
1k views

### Why does std::complex<> initialize its value to 0 upon default construction?

Doing so strikes me as a waste of time. Consider std::complex<double> *a = new std::complex<double>[1<<28]; This could be near-instantaneous ...
250 views

### Forced viscous damping in elastodynamics

I have an 2D elastodynamics problem, that is a problem which is driven by the Cauchy equation: $$\rho\ddot u-\mathrm{div}\sigma=\rho f$$ where $u$ is the displacement, $\sigma$ the Cauchy stress ...
2k views

### Compute smallest eigenvectors of a matrix

It appears that matlab's eigs is giving me bad approximations of the smallest eigenvectors of a matrix. I assume I can use some slower methods which would also be ...
43 views

### The region of allowed values ​​for solving the equation in Mathematica

In[2]:= Solve[sqrt(2x-9) == sqrt(4x+3), x] Out[2]= {{x -> -6}} But mathematically there is no solution, since sqrt (-21) is not defined. There is a flag that ...
4k views

### Problems that can be reduced to the Traveling Salesman Problem

Which search/optimization problems can be reduced to the famous "Traveling Salesman Problem"? For instance, I have a collection of N particles, in 3D, and there is a function (Van der Waals energy) ...
197 views

### MatMatMult and KSPSolve for MATMPIDENSE matrices

I'm trying to use MATMPIDENSE matrices to solve a system of type Ax=b, but I have some problems. The KSP documentation says that KSPSolve for dense matrices requires to set a 'gmres' solver and a 'lu' ...
69 views

### Are there any open standards for data exchange between components of a spatial model?

I'm just getting into climate research, and am amazed at how much poorly documented legacy code is used. This kind of thing means that models more or less have to be maintained by the people who wrote ...
4k views

### How to find QR decomposition of a rectangular matrix in overdetermined linear system solution?

While trying to find cell-centered gradients in finite volume method computation of incompressible fluid flow I get over-determined linear system. This is a well known "cell based least-square" ...
1k views

### Reporting curve-fit results in a scientific paper

(I hope this question fits this site; if not, accept my apologies). I ran a certain simulation, and got a time series y(t), t = 0, 1, ... 20. After trying some functions, I found that: ...
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### What material should I include with a journal article (or post online) in order to make my computational research reproducible?

Reproducibility has become more and more important in computational science research. (For instance, see this article by Roger Peng in Science; I'm aware of other such articles and web sites also.) ...
256 views

Can anyone point me to methods for varying $h$ in gradient estimation for noisy numerical optimization? Some programs have the user give a fixed $h$, which is used for forward-difference or central-...
832 views

### Optimal use of Strang splitting (for reaction diffusion equation)

I made a strange observation while computing the solution to a simple 1D reaction diffusion equation: $\frac{\partial}{\partial t}a=\frac{\partial^2}{\partial x^2}a-ab$ \$\frac{\partial}{\...
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### I/O Strategies for computational problems with large data sets?

My research group focuses on molecular dynamics, which obviously can generate gigabytes of data as part of a single trajectory which must then be analyzed. Several of the problems we're concerned ...
602 views

### Perron-Frobenius theorem on general real symmetric matrices

From the Perron-Frobenius theorem, it might be concluded that the spectral radius is the largest eigenvalue for positive matrices, ie, matrices with strictly positive entries. In other words, the ...
2k views

### Why can't Householder reflections diagonalize a matrix?

When computing the QR factorization in practice, one uses Householder reflections to zero out the lower portion of a matrix. I know that for computing eigenvalues of symmetric matrices, the best you ...