Reference documentation for deal.II version 9.4.1
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tensor.cc File Reference
#include <deal.II/base/tensor.h>
#include <deal.II/lac/exceptions.h>
#include <deal.II/lac/lapack_templates.h>
#include <array>

Go to the source code of this file.

## Functions

template<int dim, typename Number >
Tensor< 2, dim, Number > project_onto_orthogonal_tensors (const Tensor< 2, dim, Number > &A)

template Tensor< 2, 1, float > project_onto_orthogonal_tensors (const Tensor< 2, 1, float > &)

template Tensor< 2, 2, float > project_onto_orthogonal_tensors (const Tensor< 2, 2, float > &)

template Tensor< 2, 3, float > project_onto_orthogonal_tensors (const Tensor< 2, 3, float > &)

template Tensor< 2, 1, double > project_onto_orthogonal_tensors (const Tensor< 2, 1, double > &)

template Tensor< 2, 2, double > project_onto_orthogonal_tensors (const Tensor< 2, 2, double > &)

template Tensor< 2, 3, double > project_onto_orthogonal_tensors (const Tensor< 2, 3, double > &)

## ◆ project_onto_orthogonal_tensors() [1/7]

template<int dim, typename Number >
 Tensor< 2, dim, Number > project_onto_orthogonal_tensors ( const Tensor< 2, dim, Number > & A )

Return the nearest orthogonal matrix $$\hat {\mathbf A}=\mathbf U \mathbf{V}^T$$ by combining the products of the singular value decomposition (SVD) $${\mathbf A}=\mathbf U \mathbf S \mathbf V^T$$ for a given input $${\mathbf A}$$, effectively replacing $$\mathbf S$$ with the identity matrix.

This is a (nonlinear) projection operation since when applied twice, we have $$\hat{\hat{\mathbf A}}=\hat{\mathbf A}$$ as is easy to see. (That is because the SVD of $$\hat {\mathbf A}$$ is simply $$\mathbf U \mathbf I \mathbf{V}^T$$.) Furthermore, $$\hat {\mathbf A}$$ is really an orthogonal matrix because orthogonal matrices have to satisfy $${\hat {\mathbf A}}^T \hat {\mathbf A}={\mathbf I}$$, which here implies that

\begin{align*} {\hat {\mathbf A}}^T \hat {\mathbf A} &= \left(\mathbf U \mathbf{V}^T\right)^T\left(\mathbf U \mathbf{V}^T\right) \\ &= \mathbf V \mathbf{U}^T \mathbf U \mathbf{V}^T \\ &= \mathbf V \left(\mathbf{U}^T \mathbf U\right) \mathbf{V}^T \\ &= \mathbf V \mathbf I \mathbf{V}^T \\ &= \mathbf V \mathbf{V}^T \\ &= \mathbf I \end{align*}

due to the fact that the $$\mathbf U$$ and $$\mathbf V$$ factors that come out of the SVD are themselves orthogonal matrices.

Parameters
 A The tensor for which to find the closest orthogonal tensor.
Template Parameters
 Number The type used to store the entries of the tensor. Must be either float or double.
Precondition
In order to use this function, this program must be linked with the LAPACK library.
A must not be singular. This is because, conceptually, the problem to be solved here is trying to find a matrix $$\hat{\mathbf A}$$ that minimizes some kind of distance from $$\mathbf A$$ while satisfying the quadratic constraint $${\hat {\mathbf A}}^T \hat {\mathbf A}={\mathbf I}$$. This is not so dissimilar to the kind of problem where one wants to find a vector $$\hat{\mathbf x}\in{\mathbb R}^n$$ that minimizes the quadratic objective function $$\|\hat {\mathbf x} - \mathbf x\|^2$$ for a given $$\mathbf x$$ subject to the constraint $$\|\mathbf x\|^2=1$$ – in other words, we are seeking the point $$\hat{\mathbf x}$$ on the unit sphere that is closest to $$\mathbf x$$. This problem has a solution for all $$\mathbf x$$ except if $$\mathbf x=0$$. The corresponding condition for the problem we are considering here is that $$\mathbf A$$ must not have a zero eigenvalue.

Definition at line 71 of file tensor.cc.

## ◆ project_onto_orthogonal_tensors() [2/7]

 template Tensor< 2, 1, float > project_onto_orthogonal_tensors ( const Tensor< 2, 1, float > & )

## ◆ project_onto_orthogonal_tensors() [3/7]

 template Tensor< 2, 2, float > project_onto_orthogonal_tensors ( const Tensor< 2, 2, float > & )

## ◆ project_onto_orthogonal_tensors() [4/7]

 template Tensor< 2, 3, float > project_onto_orthogonal_tensors ( const Tensor< 2, 3, float > & )

## ◆ project_onto_orthogonal_tensors() [5/7]

 template Tensor< 2, 1, double > project_onto_orthogonal_tensors ( const Tensor< 2, 1, double > & )

## ◆ project_onto_orthogonal_tensors() [6/7]

 template Tensor< 2, 2, double > project_onto_orthogonal_tensors ( const Tensor< 2, 2, double > & )

## ◆ project_onto_orthogonal_tensors() [7/7]

 template Tensor< 2, 3, double > project_onto_orthogonal_tensors ( const Tensor< 2, 3, double > & )