org.apache.commons.math3.linear

## Class SchurTransformer

- java.lang.Object
- - org.apache.commons.math3.linear.SchurTransformer

     class SchurTransformer
    extends java.lang.Object

Class transforming a general real matrix to Schur form.

A m × m matrix A can be written as the product of three matrices: A = P × T × PT with P an orthogonal matrix and T an quasi-triangular matrix. Both P and T are m × m matrices.

Transformation to Schur form is often not a goal by itself, but it is an intermediate step in more general decomposition algorithms like `eigen decomposition`. This class is therefore intended for internal use by the library and is not public. As a consequence of this explicitly limited scope, many methods directly returns references to internal arrays, not copies.

This class is based on the method hqr2 in class EigenvalueDecomposition from the JAMA library.

Since:
  3.1
Version:
  $Id: SchurTransformer.java 1389129 2012-09-23 19:34:02Z tn $
See Also:
  Schur Decomposition - MathWorld, Schur Decomposition - Wikipedia, Householder Transformations

### Nested Class Summary

Nested Classes
`private static class` `SchurTransformer.ShiftInfo` Internal data structure holding the current shift information.

### Field Summary

Fields
`private RealMatrix` `cachedP` Cached value of P.
`private RealMatrix` `cachedPt` Cached value of PT.
`private RealMatrix` `cachedT` Cached value of T.
`private double` `epsilon` Epsilon criteria taken from JAMA code (originally was 2^-52).
`private double[][]` `matrixP` P matrix.
`private double[][]` `matrixT` T matrix.
`private static int` `MAX_ITERATIONS` Maximum allowed iterations for convergence of the transformation.

### Constructor Summary

Constructors
Constructor and Description
`SchurTransformer(RealMatrix matrix)` Build the transformation to Schur form of a general real matrix.

### Method Summary

Methods
`private void` `computeShift(int l, int idx, int iteration, SchurTransformer.ShiftInfo shift)` Compute the shift for the current iteration.
`private int` `findSmallSubDiagonalElement(int startIdx, double norm)` Find the first small sub-diagonal element and returns its index.
`private double` `getNorm()` Computes the L1 norm of the (quasi-)triangular matrix T.
`RealMatrix` `getP()` Returns the matrix P of the transform.
`RealMatrix` `getPT()` Returns the transpose of the matrix P of the transform.
`RealMatrix` `getT()` Returns the quasi-triangular Schur matrix T of the transform.
`private int` `initQRStep(int il, int iu, SchurTransformer.ShiftInfo shift, double[] hVec)` Initialize the householder vectors for the QR step.
`private void` `performDoubleQRStep(int il, int im, int iu, SchurTransformer.ShiftInfo shift, double[] hVec)` Perform a double QR step involving rows l:idx and columns m:n
`private void` `transform()` Transform original matrix to Schur form.

### Methods inherited from class java.lang.Object

`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

### Field Detail

#### MAX_ITERATIONS

    private static final int MAX_ITERATIONS

Maximum allowed iterations for convergence of the transformation.

See Also:
  Constant Field Values

#### matrixP

    private final double[][] matrixP

P matrix.

#### matrixT

    private final double[][] matrixT

T matrix.

#### cachedP

    private RealMatrix cachedP

Cached value of P.

#### cachedT

    private RealMatrix cachedT

Cached value of T.

#### cachedPt

    private RealMatrix cachedPt

Cached value of PT.

#### epsilon

    private final double epsilon

Epsilon criteria taken from JAMA code (originally was 2^-52).

### Constructor Detail

#### SchurTransformer

    public SchurTransformer(RealMatrix matrix)

Build the transformation to Schur form of a general real matrix.

Parameters:
  `matrix` - matrix to transform
Throws:
  `NonSquareMatrixException` - if the matrix is not square

### Method Detail

#### getP

    public RealMatrix getP()

Returns the matrix P of the transform.

P is an orthogonal matrix, i.e. its inverse is also its transpose.

Returns:
  the P matrix

#### getPT

    public RealMatrix getPT()

Returns the transpose of the matrix P of the transform.

P is an orthogonal matrix, i.e. its inverse is also its transpose.

Returns:
  the transpose of the P matrix

#### getT

    public RealMatrix getT()

Returns the quasi-triangular Schur matrix T of the transform.

Returns:
  the T matrix

#### transform

    private void transform()

Transform original matrix to Schur form.

Throws:
  `MaxCountExceededException` - if the transformation does not converge

#### getNorm

    private double getNorm()

Computes the L1 norm of the (quasi-)triangular matrix T.

Returns:
  the L1 norm of matrix T

#### findSmallSubDiagonalElement

    private int findSmallSubDiagonalElement(int startIdx,
                                  double norm)

Find the first small sub-diagonal element and returns its index.

Parameters:
  `startIdx` - the starting index for the search
  `norm` - the L1 norm of the matrix
Returns:
  the index of the first small sub-diagonal element

#### computeShift

    private void computeShift(int l,
                    int idx,
                    int iteration,
                    SchurTransformer.ShiftInfo shift)

Compute the shift for the current iteration.

Parameters:
  `l` - the index of the small sub-diagonal element
  `idx` - the current eigenvalue index
  `iteration` - the current iteration
  `shift` - holder for shift information

#### initQRStep

    private int initQRStep(int il,
                 int iu,
                 SchurTransformer.ShiftInfo shift,
                 double[] hVec)

Initialize the householder vectors for the QR step.

Parameters:
  `il` - the index of the small sub-diagonal element
  `iu` - the current eigenvalue index
  `shift` - shift information holder
  `hVec` - the initial houseHolder vector
Returns:
  the start index for the QR step

#### performDoubleQRStep

    private void performDoubleQRStep(int il,
                           int im,
                           int iu,
                           SchurTransformer.ShiftInfo shift,
                           double[] hVec)

Perform a double QR step involving rows l:idx and columns m:n

Parameters:
  `il` - the index of the small sub-diagonal element
  `im` - the start index for the QR step
  `iu` - the current eigenvalue index
  `shift` - shift information holder
  `hVec` - the initial houseHolder vector