# Questions tagged [symmetry]

For questions about exploiting symmetries to solve computational problems. This could include seeking an algorithm for symmetric matrices or finding machine learning descriptors that are invariant under certain symmetry operations.

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### Symmetrization of Laplacian Matrix Operator (finite volumes)

The aim is to construct symmetric Laplacian matrix $L$ for 2D square domain. I have the following discretization of second derivatives on non-uniform grid (will skip the steps of derivation, any ...
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### Compatibility condition for Poisson equation in cylindrical symmetry

I'm trying to implement multigrid approach for a Poisson equation $\frac{1}{r}\frac{\partial}{\partial r}\left( r \frac{\partial H}{\partial r} \right) = f$ with all Neumann boundary conditions. ...
55 views

### Under what specific circumstances can a matrix lose symmetry?

This is a general question: In this blog post, the author points out that under a unitarily invariant norm, the closest symmetric matrix to the square matrix $M$ is $\frac 1 2 (M + M^T)$. The general ...
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### Methods to improve the efficiency and the memory requirement of LU factorization for complex symmetric system matrix

I want to solve a linear set of equations (Ax=b) using LU decomposition. My "A" matrix is a complex matrix which is ...
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### How property-invariance is imposed to neural nets?

I was wondering how specific symmetries or constraints such as property-invariance transformation are imposed on any (deep) neural net when they are trained. I'll appreciate it if anyone can aware me ...
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### Does a symmetric positive definite matrix also have $\mathbf{A} = \mathbf{L}^T\mathbf{L}$ (where $\mathbf{L}$ is a lower triangular matrix)?

As we know, for a symmetric positive definite (SPD) matrix $\mathbf{A}$, there is a theorem about the Cholesky factorization $\mathbf{A}= \mathbf{L}\mathbf{L}^T$, where $\mathbf{L}$ is a lower ...
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### How can Navier--Stokes equations have asymmetric solutions such as Karman vortex streets

The Navier--Stokes equations are axially symmetric, so with symmetric boundary conditions, how can features such as Karman vortex streets develop? I understand that in reality symmetry does never ...
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### Conservation violation in axisymmetric Diffusion Equation

1d diffusion equation Integrating the diffusion equation, $$\frac{\partial u}{\partial t} = D \frac{\partial^2 u}{\partial x^2},$$ with a constant diffusion coefficient D using forward Euler for ...
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### CG question: is symmetry always necessary?

Consider the 1D Poisson equation $$\nabla^2 u = f.$$ Using finite difference method on cell corner data and a uniform grid with ghost points, I think we can write the system of equations with Neumann ...
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### Mapping from n x n complex symmetric tridiagonal to 2n x 2n real symmetric tridiagonal

In my program I have a complex symmetric tridiagonal matrix. In order to perform some algorithmic optimizations I am searching for a (ideally linear) mapping from $n\times n$ complex symmetric ...
663 views

### Why does conjugate gradient work with this nonsymmetric preconditioner?

In this previous thread the following multiplicative way to combine symmetric preconditioners $P_1$ and $P_2$ for the symmetric system $Ax=b$ was suggested: \begin{align} P_\text{combo}^{-1} :=& ...
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### Which numerical methods preserve time reversal symmetry?

If I have a physical system which contains a time reversal symmetry (for example a Hamiltonian $H(x,p)=p^2/2m + V(x)$ with $V(x)$ real) and I want to solve the differential equations which describe ...
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### (Fortran) Integrating/summing over complicated 3D domain

I have some function $F(k_x,k_y,k_z)$ that I wish to numerically integrate over a polygon domain - physically, I am integrating over the first Brillouin Zone (BZ) of the FCC lattice (a truncated ...
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1 vote
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### Is there congruent transform implementation for dense symmetric matrix in Eigen(C++)?

I need to determine whether a real dense symmetric matrix is positive definite or not. One possible way is to obtain all the eigen values and check the sign of the minimum eigen value but requires ...