Eigen/src/Eigenvalues/SelfAdjointEigenSolver.h - mirror - Git at Google

 // This file is part of Eigen, a lightweight C++ template library
 // for linear algebra.
 //
 // Copyright (C) 2008-2010 Gael Guennebaud <g.gael@free.fr>
 // Copyright (C) 2010 Jitse Niesen <jitse@maths.leeds.ac.uk>
 //
 // Eigen is free software; you can redistribute it and/or
 // modify it under the terms of the GNU Lesser General Public
 // License as published by the Free Software Foundation; either
 // version 3 of the License, or (at your option) any later version.
 //
 // Alternatively, you can redistribute it and/or
 // modify it under the terms of the GNU General Public License as
 // published by the Free Software Foundation; either version 2 of
 // the License, or (at your option) any later version.
 //
 // Eigen is distributed in the hope that it will be useful, but WITHOUT ANY
 // WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
 // FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License or the
 // GNU General Public License for more details.
 //
 // You should have received a copy of the GNU Lesser General Public
 // License and a copy of the GNU General Public License along with
 // Eigen. If not, see <http://www.gnu.org/licenses/>.

 #ifndef EIGEN_SELFADJOINTEIGENSOLVER_H
 #define EIGEN_SELFADJOINTEIGENSOLVER_H

 #include "./EigenvaluesCommon.h"
 #include "./Tridiagonalization.h"

 /** \eigenvalues_module \ingroup Eigenvalues_Module
   * \nonstableyet
   *
   * \class SelfAdjointEigenSolver
   *
   * \brief Computes eigenvalues and eigenvectors of selfadjoint matrices
   *
   * \tparam _MatrixType the type of the matrix of which we are computing the
   * eigendecomposition; this is expected to be an instantiation of the Matrix
   * class template.
   *
   * A matrix \f$ A \f$ is selfadjoint if it equals its adjoint. For real
   * matrices, this means that the matrix is symmetric: it equals its
   * transpose. This class computes the eigenvalues and eigenvectors of a
   * selfadjoint matrix. These are the scalars \f$ \lambda \f$ and vectors
   * \f$ v \f$ such that \f$ Av = \lambda v \f$.  The eigenvalues of a
   * selfadjoint matrix are always real. If \f$ D \f$ is a diagonal matrix with
   * the eigenvalues on the diagonal, and \f$ V \f$ is a matrix with the
   * eigenvectors as its columns, then \f$ A = V D V^{-1} \f$ (for selfadjoint
   * matrices, the matrix \f$ V \f$ is always invertible). This is called the
   * eigendecomposition.
   *
   * The algorithm exploits the fact that the matrix is selfadjoint, making it
   * faster and more accurate than the general purpose eigenvalue algorithms
   * implemented in EigenSolver and ComplexEigenSolver.
   *
   * Only the \b lower \b triangular \b part of the input matrix is referenced.
   *
   * Call the function compute() to compute the eigenvalues and eigenvectors of
   * a given matrix. Alternatively, you can use the
   * SelfAdjointEigenSolver(const MatrixType&, bool) constructor which computes
   * the eigenvalues and eigenvectors at construction time. Once the eigenvalue
   * and eigenvectors are computed, they can be retrieved with the eigenvalues()
   * and eigenvectors() functions.
   *
   * The documentation for SelfAdjointEigenSolver(const MatrixType&, bool)
   * contains an example of the typical use of this class.
   *
   * To solve the \em generalized eigenvalue problem \f$ Av = \lambda Bv \f$ and
   * the likes, see the class GeneralizedSelfAdjointEigenSolver.
   *
   * \sa MatrixBase::eigenvalues(), class EigenSolver, class ComplexEigenSolver
   */
 template<typename _MatrixType> class SelfAdjointEigenSolver
 {
   public:

     typedef _MatrixType MatrixType;
     enum {
       Size = MatrixType::RowsAtCompileTime,
       ColsAtCompileTime = MatrixType::ColsAtCompileTime,
       Options = MatrixType::Options,
       MaxColsAtCompileTime = MatrixType::MaxColsAtCompileTime
     };

     /** \brief Scalar type for matrices of type \p _MatrixType. */
     typedef typename MatrixType::Scalar Scalar;
     typedef typename MatrixType::Index Index;

     /** \brief Real scalar type for \p _MatrixType.
       *
       * This is just \c Scalar if #Scalar is real (e.g., \c float or
       * \c double), and the type of the real part of \c Scalar if #Scalar is
       * complex.
       */
     typedef typename NumTraits<Scalar>::Real RealScalar;

     /** \brief Type for vector of eigenvalues as returned by eigenvalues().
       *
       * This is a column vector with entries of type #RealScalar.
       * The length of the vector is the size of \p _MatrixType.
       */
     typedef typename ei_plain_col_type<MatrixType, RealScalar>::type RealVectorType;
     typedef Tridiagonalization<MatrixType> TridiagonalizationType;

     /** \brief Default constructor for fixed-size matrices.
       *
       * The default constructor is useful in cases in which the user intends to
       * perform decompositions via compute(const MatrixType&, bool) or
       * compute(const MatrixType&, const MatrixType&, bool). This constructor
       * can only be used if \p _MatrixType is a fixed-size matrix; use
       * SelfAdjointEigenSolver(Index) for dynamic-size matrices.
       *
       * Example: \include SelfAdjointEigenSolver_SelfAdjointEigenSolver.cpp
       * Output: \verbinclude SelfAdjointEigenSolver_SelfAdjointEigenSolver.out
       */
     SelfAdjointEigenSolver()
         : m_eivec(),
           m_eivalues(),
           m_subdiag(),
           m_isInitialized(false)
     { }

     /** \brief Constructor, pre-allocates memory for dynamic-size matrices.
       *
       * \param [in]  size  Positive integer, size of the matrix whose
       * eigenvalues and eigenvectors will be computed.
       *
       * This constructor is useful for dynamic-size matrices, when the user
       * intends to perform decompositions via compute(const MatrixType&, bool)
       * or compute(const MatrixType&, const MatrixType&, bool). The \p size
       * parameter is only used as a hint. It is not an error to give a wrong
       * \p size, but it may impair performance.
       *
       * \sa compute(const MatrixType&, bool) for an example
       */
     SelfAdjointEigenSolver(Index size)
         : m_eivec(size, size),
           m_eivalues(size),
           m_subdiag(size > 1 ? size - 1 : 1),
           m_isInitialized(false)
     {}

     /** \brief Constructor; computes eigendecomposition of given matrix.
       *
       * \param[in]  matrix  Selfadjoint matrix whose eigendecomposition is to
       *    be computed. Only the lower triangular part of the matrix is referenced.
       * \param[in]  options Can be ComputeEigenvectors (default) or EigenvaluesOnly.
       *
       * This constructor calls compute(const MatrixType&, bool) to compute the
       * eigenvalues of the matrix \p matrix. The eigenvectors are computed if
       * \p options equals ComputeEigenvectors.
       *
       * Example: \include SelfAdjointEigenSolver_SelfAdjointEigenSolver_MatrixType.cpp
       * Output: \verbinclude SelfAdjointEigenSolver_SelfAdjointEigenSolver_MatrixType.out
       *
       * \sa compute(const MatrixType&, bool),
       *     SelfAdjointEigenSolver(const MatrixType&, const MatrixType&, bool)
       */
     SelfAdjointEigenSolver(const MatrixType& matrix, int options = ComputeEigenvectors)
       : m_eivec(matrix.rows(), matrix.cols()),
         m_eivalues(matrix.cols()),
         m_subdiag(matrix.rows() > 1 ? matrix.rows() - 1 : 1),
         m_isInitialized(false)
     {
       compute(matrix, options);
     }

     /** \brief Computes eigendecomposition of given matrix.
       *
       * \param[in]  matrix  Selfadjoint matrix whose eigendecomposition is to
       *    be computed. Only the lower triangular part of the matrix is referenced.
       * \param[in]  options Can be ComputeEigenvectors (default) or EigenvaluesOnly.
       * \returns    Reference to \c *this
       *
       * This function computes the eigenvalues of \p matrix.  The eigenvalues()
       * function can be used to retrieve them.  If \p options equals ComputeEigenvectors,
       * then the eigenvectors are also computed and can be retrieved by
       * calling eigenvectors().
       *
       * This implementation uses a symmetric QR algorithm. The matrix is first
       * reduced to tridiagonal form using the Tridiagonalization class. The
       * tridiagonal matrix is then brought to diagonal form with implicit
       * symmetric QR steps with Wilkinson shift. Details can be found in
       * Section 8.3 of Golub \& Van Loan, <i>%Matrix Computations</i>.
       *
       * The cost of the computation is about \f$ 9n^3 \f$ if the eigenvectors
       * are required and \f$ 4n^3/3 \f$ if they are not required.
       *
       * This method reuses the memory in the SelfAdjointEigenSolver object that
       * was allocated when the object was constructed, if the size of the
       * matrix does not change.
       *
       * Example: \include SelfAdjointEigenSolver_compute_MatrixType.cpp
       * Output: \verbinclude SelfAdjointEigenSolver_compute_MatrixType.out
       *
       * \sa SelfAdjointEigenSolver(const MatrixType&, bool)
       */
     SelfAdjointEigenSolver& compute(const MatrixType& matrix, int options = ComputeEigenvectors);

     /** \brief Returns the eigenvectors of given matrix (pencil).
       *
       * \returns  A const reference to the matrix whose columns are the eigenvectors.
       *
       * \pre The eigenvectors have been computed before.
       *
       * Column \f$ k \f$ of the returned matrix is an eigenvector corresponding
       * to eigenvalue number \f$ k \f$ as returned by eigenvalues().  The
       * eigenvectors are normalized to have (Euclidean) norm equal to one. If
       * this object was used to solve the eigenproblem for the selfadjoint
       * matrix \f$ A \f$, then the matrix returned by this function is the
       * matrix \f$ V \f$ in the eigendecomposition \f$ A = V D V^{-1} \f$.
       *
       * Example: \include SelfAdjointEigenSolver_eigenvectors.cpp
       * Output: \verbinclude SelfAdjointEigenSolver_eigenvectors.out
       *
       * \sa eigenvalues()
       */
     const MatrixType& eigenvectors() const
     {
       ei_assert(m_isInitialized && "SelfAdjointEigenSolver is not initialized.");
       ei_assert(m_eigenvectorsOk && "The eigenvectors have not been computed together with the eigenvalues.");
       return m_eivec;
     }

     /** \brief Returns the eigenvalues of given matrix (pencil).
       *
       * \returns A const reference to the column vector containing the eigenvalues.
       *
       * \pre The eigenvalues have been computed before.
       *
       * The eigenvalues are repeated according to their algebraic multiplicity,
       * so there are as many eigenvalues as rows in the matrix.
       *
       * Example: \include SelfAdjointEigenSolver_eigenvalues.cpp
       * Output: \verbinclude SelfAdjointEigenSolver_eigenvalues.out
       *
       * \sa eigenvectors(), MatrixBase::eigenvalues()
       */
     const RealVectorType& eigenvalues() const
     {
       ei_assert(m_isInitialized && "SelfAdjointEigenSolver is not initialized.");
       return m_eivalues;
     }

     /** \brief Computes the positive-definite square root of the matrix.
       *
       * \returns the positive-definite square root of the matrix
       *
       * \pre The eigenvalues and eigenvectors of a positive-definite matrix
       * have been computed before.
       *
       * The square root of a positive-definite matrix \f$ A \f$ is the
       * positive-definite matrix whose square equals \f$ A \f$. This function
       * uses the eigendecomposition \f$ A = V D V^{-1} \f$ to compute the
       * square root as \f$ A^{1/2} = V D^{1/2} V^{-1} \f$.
       *
       * Example: \include SelfAdjointEigenSolver_operatorSqrt.cpp
       * Output: \verbinclude SelfAdjointEigenSolver_operatorSqrt.out
       *
       * \sa operatorInverseSqrt(),
       *     \ref MatrixFunctions_Module "MatrixFunctions Module"
       */
     MatrixType operatorSqrt() const
     {
       ei_assert(m_isInitialized && "SelfAdjointEigenSolver is not initialized.");
       ei_assert(m_eigenvectorsOk && "The eigenvectors have not been computed together with the eigenvalues.");
       return m_eivec * m_eivalues.cwiseSqrt().asDiagonal() * m_eivec.adjoint();
     }

     /** \brief Computes the inverse square root of the matrix.
       *
       * \returns the inverse positive-definite square root of the matrix
       *
       * \pre The eigenvalues and eigenvectors of a positive-definite matrix
       * have been computed before.
       *
       * This function uses the eigendecomposition \f$ A = V D V^{-1} \f$ to
       * compute the inverse square root as \f$ V D^{-1/2} V^{-1} \f$. This is
       * cheaper than first computing the square root with operatorSqrt() and
       * then its inverse with MatrixBase::inverse().
       *
       * Example: \include SelfAdjointEigenSolver_operatorInverseSqrt.cpp
       * Output: \verbinclude SelfAdjointEigenSolver_operatorInverseSqrt.out
       *
       * \sa operatorSqrt(), MatrixBase::inverse(),
       *     \ref MatrixFunctions_Module "MatrixFunctions Module"
       */
     MatrixType operatorInverseSqrt() const
     {
       ei_assert(m_isInitialized && "SelfAdjointEigenSolver is not initialized.");
       ei_assert(m_eigenvectorsOk && "The eigenvectors have not been computed together with the eigenvalues.");
       return m_eivec * m_eivalues.cwiseInverse().cwiseSqrt().asDiagonal() * m_eivec.adjoint();
     }

     /** \brief Reports whether previous computation was successful.
       *
       * \returns \c Success if computation was succesful, \c NoConvergence otherwise.
       */
     ComputationInfo info() const
     {
       ei_assert(m_isInitialized && "SelfAdjointEigenSolver is not initialized.");
       return m_info;
     }

     /** \brief Maximum number of iterations.
       *
       * Maximum number of iterations allowed for an eigenvalue to converge.
       */
     static const int m_maxIterations = 30;

   protected:
     MatrixType m_eivec;
     RealVectorType m_eivalues;
     typename TridiagonalizationType::SubDiagonalType m_subdiag;
     ComputationInfo m_info;
     bool m_isInitialized;
     bool m_eigenvectorsOk;
 };

 /** \internal
   *
   * \eigenvalues_module \ingroup Eigenvalues_Module
   *
   * Performs a QR step on a tridiagonal symmetric matrix represented as a
   * pair of two vectors \a diag and \a subdiag.
   *
   * \param matA the input selfadjoint matrix
   * \param hCoeffs returned Householder coefficients
   *
   * For compilation efficiency reasons, this procedure does not use eigen expression
   * for its arguments.
   *
   * Implemented from Golub's "Matrix Computations", algorithm 8.3.2:
   * "implicit symmetric QR step with Wilkinson shift"
   */
 template<typename RealScalar, typename Scalar, typename Index>
 static void ei_tridiagonal_qr_step(RealScalar* diag, RealScalar* subdiag, Index start, Index end, Scalar* matrixQ, Index n);

 template<typename MatrixType>
 SelfAdjointEigenSolver<MatrixType>& SelfAdjointEigenSolver<MatrixType>
 ::compute(const MatrixType& matrix, int options)
 {
   ei_assert(matrix.cols() == matrix.rows());
   ei_assert((options&~(EigVecMask|GenEigMask))==0
           && (options&EigVecMask)!=EigVecMask
           && "invalid option parameter");
   bool computeEigenvectors = (options&ComputeEigenvectors)==ComputeEigenvectors;
   Index n = matrix.cols();
   m_eivalues.resize(n,1);

   if(n==1)
   {
     m_eivalues.coeffRef(0,0) = ei_real(matrix.coeff(0,0));
     if(computeEigenvectors)
       m_eivec.setOnes();
     m_info = Success;
     m_isInitialized = true;
     m_eigenvectorsOk = computeEigenvectors;
     return *this;
   }

   // declare some aliases
   RealVectorType& diag = m_eivalues;
   MatrixType& mat = m_eivec;

   mat = matrix;
   m_subdiag.resize(n-1);
   ei_tridiagonalization_inplace(mat, diag, m_subdiag, computeEigenvectors);

   Index end = n-1;
   Index start = 0;
   Index iter = 0; // number of iterations we are working on one element

   while (end>0)
   {
     for (Index i = start; i<end; ++i)
       if (ei_isMuchSmallerThan(ei_abs(m_subdiag[i]),(ei_abs(diag[i])+ei_abs(diag[i+1]))))
         m_subdiag[i] = 0;

     // find the largest unreduced block
     while (end>0 && m_subdiag[end-1]==0)
     {
       iter = 0;
       end--;
     }
     if (end<=0)
       break;

     // if we spent too many iterations on the current element, we give up
     iter++;
     if(iter > m_maxIterations) break;

     start = end - 1;
     while (start>0 && m_subdiag[start-1]!=0)
       start--;

     ei_tridiagonal_qr_step(diag.data(), m_subdiag.data(), start, end, computeEigenvectors ? m_eivec.data() : (Scalar*)0, n);
   }

   if (iter <= m_maxIterations)
     m_info = Success;
   else
     m_info = NoConvergence;

   // Sort eigenvalues and corresponding vectors.
   // TODO make the sort optional ?
   // TODO use a better sort algorithm !!
   if (m_info == Success)
   {
     for (Index i = 0; i < n-1; ++i)
     {
       Index k;
       m_eivalues.segment(i,n-i).minCoeff(&k);
       if (k > 0)
       {
         std::swap(m_eivalues[i], m_eivalues[k+i]);
         if(computeEigenvectors)
           m_eivec.col(i).swap(m_eivec.col(k+i));
       }
     }
   }

   m_isInitialized = true;
   m_eigenvectorsOk = computeEigenvectors;
   return *this;
 }

 template<typename RealScalar, typename Scalar, typename Index>
 static void ei_tridiagonal_qr_step(RealScalar* diag, RealScalar* subdiag, Index start, Index end, Scalar* matrixQ, Index n)
 {
   RealScalar td = (diag[end-1] - diag[end])*RealScalar(0.5);
   RealScalar e2 = ei_abs2(subdiag[end-1]);
   RealScalar mu = diag[end] - e2 / (td + (td>0 ? 1 : -1) * ei_sqrt(td*td + e2));
   RealScalar x = diag[start] - mu;
   RealScalar z = subdiag[start];

   for (Index k = start; k < end; ++k)
   {
     PlanarRotation<RealScalar> rot;
     rot.makeGivens(x, z);

     // do T = G' T G
     RealScalar sdk = rot.s() * diag[k] + rot.c() * subdiag[k];
     RealScalar dkp1 = rot.s() * subdiag[k] + rot.c() * diag[k+1];

     diag[k] = rot.c() * (rot.c() * diag[k] - rot.s() * subdiag[k]) - rot.s() * (rot.c() * subdiag[k] - rot.s() * diag[k+1]);
     diag[k+1] = rot.s() * sdk + rot.c() * dkp1;
     subdiag[k] = rot.c() * sdk - rot.s() * dkp1;

     if (k > start)
       subdiag[k - 1] = rot.c() * subdiag[k-1] - rot.s() * z;

     x = subdiag[k];

     if (k < end - 1)
     {
       z = -rot.s() * subdiag[k+1];
       subdiag[k + 1] = rot.c() * subdiag[k+1];
     }

     // apply the givens rotation to the unit matrix Q = Q * G
     if (matrixQ)
     {
       Map<Matrix<Scalar,Dynamic,Dynamic> > q(matrixQ,n,n);
       q.applyOnTheRight(k,k+1,rot);
     }
   }
 }

 #endif // EIGEN_SELFADJOINTEIGENSOLVER_H
	// This file is part of Eigen, a lightweight C++ template library
	// for linear algebra.
	//
	// Copyright (C) 2008-2010 Gael Guennebaud <g.gael@free.fr>
	// Copyright (C) 2010 Jitse Niesen <jitse@maths.leeds.ac.uk>
	//
	// Eigen is free software; you can redistribute it and/or
	// modify it under the terms of the GNU Lesser General Public
	// License as published by the Free Software Foundation; either
	// version 3 of the License, or (at your option) any later version.
	//
	// Alternatively, you can redistribute it and/or
	// modify it under the terms of the GNU General Public License as
	// published by the Free Software Foundation; either version 2 of
	// the License, or (at your option) any later version.
	//
	// Eigen is distributed in the hope that it will be useful, but WITHOUT ANY
	// WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
	// FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License or the
	// GNU General Public License for more details.
	//
	// You should have received a copy of the GNU Lesser General Public
	// License and a copy of the GNU General Public License along with
	// Eigen. If not, see <http://www.gnu.org/licenses/>.

	#ifndef EIGEN_SELFADJOINTEIGENSOLVER_H
	#define EIGEN_SELFADJOINTEIGENSOLVER_H

	#include "./EigenvaluesCommon.h"
	#include "./Tridiagonalization.h"

	/** \eigenvalues_module \ingroup Eigenvalues_Module
	* \nonstableyet
	*
	* \class SelfAdjointEigenSolver
	*
	* \brief Computes eigenvalues and eigenvectors of selfadjoint matrices
	*
	* \tparam _MatrixType the type of the matrix of which we are computing the
	* eigendecomposition; this is expected to be an instantiation of the Matrix
	* class template.
	*
	* A matrix \f$ A \f$ is selfadjoint if it equals its adjoint. For real
	* matrices, this means that the matrix is symmetric: it equals its
	* transpose. This class computes the eigenvalues and eigenvectors of a
	* selfadjoint matrix. These are the scalars \f$ \lambda \f$ and vectors
	* \f$ v \f$ such that \f$ Av = \lambda v \f$. The eigenvalues of a
	* selfadjoint matrix are always real. If \f$ D \f$ is a diagonal matrix with
	* the eigenvalues on the diagonal, and \f$ V \f$ is a matrix with the
	* eigenvectors as its columns, then \f$ A = V D V^{-1} \f$ (for selfadjoint
	* matrices, the matrix \f$ V \f$ is always invertible). This is called the
	* eigendecomposition.
	*
	* The algorithm exploits the fact that the matrix is selfadjoint, making it
	* faster and more accurate than the general purpose eigenvalue algorithms
	* implemented in EigenSolver and ComplexEigenSolver.
	*
	* Only the \b lower \b triangular \b part of the input matrix is referenced.
	*
	* Call the function compute() to compute the eigenvalues and eigenvectors of
	* a given matrix. Alternatively, you can use the
	* SelfAdjointEigenSolver(const MatrixType&, bool) constructor which computes
	* the eigenvalues and eigenvectors at construction time. Once the eigenvalue
	* and eigenvectors are computed, they can be retrieved with the eigenvalues()
	* and eigenvectors() functions.
	*
	* The documentation for SelfAdjointEigenSolver(const MatrixType&, bool)
	* contains an example of the typical use of this class.
	*
	* To solve the \em generalized eigenvalue problem \f$ Av = \lambda Bv \f$ and
	* the likes, see the class GeneralizedSelfAdjointEigenSolver.
	*
	* \sa MatrixBase::eigenvalues(), class EigenSolver, class ComplexEigenSolver
	*/
	template<typename _MatrixType> class SelfAdjointEigenSolver
	{
	public:

	typedef _MatrixType MatrixType;
	enum {
	Size = MatrixType::RowsAtCompileTime,
	ColsAtCompileTime = MatrixType::ColsAtCompileTime,
	Options = MatrixType::Options,
	MaxColsAtCompileTime = MatrixType::MaxColsAtCompileTime
	};

	/** \brief Scalar type for matrices of type \p _MatrixType. */
	typedef typename MatrixType::Scalar Scalar;
	typedef typename MatrixType::Index Index;

	/** \brief Real scalar type for \p _MatrixType.
	*
	* This is just \c Scalar if #Scalar is real (e.g., \c float or
	* \c double), and the type of the real part of \c Scalar if #Scalar is
	* complex.
	*/
	typedef typename NumTraits<Scalar>::Real RealScalar;

	/** \brief Type for vector of eigenvalues as returned by eigenvalues().
	*
	* This is a column vector with entries of type #RealScalar.
	* The length of the vector is the size of \p _MatrixType.
	*/
	typedef typename ei_plain_col_type<MatrixType, RealScalar>::type RealVectorType;
	typedef Tridiagonalization<MatrixType> TridiagonalizationType;

	/** \brief Default constructor for fixed-size matrices.
	*
	* The default constructor is useful in cases in which the user intends to
	* perform decompositions via compute(const MatrixType&, bool) or
	* compute(const MatrixType&, const MatrixType&, bool). This constructor
	* can only be used if \p _MatrixType is a fixed-size matrix; use
	* SelfAdjointEigenSolver(Index) for dynamic-size matrices.
	*
	* Example: \include SelfAdjointEigenSolver_SelfAdjointEigenSolver.cpp
	* Output: \verbinclude SelfAdjointEigenSolver_SelfAdjointEigenSolver.out
	*/
	SelfAdjointEigenSolver()
	: m_eivec(),
	m_eivalues(),
	m_subdiag(),
	m_isInitialized(false)
	{ }

	/** \brief Constructor, pre-allocates memory for dynamic-size matrices.
	*
	* \param [in] size Positive integer, size of the matrix whose
	* eigenvalues and eigenvectors will be computed.
	*
	* This constructor is useful for dynamic-size matrices, when the user
	* intends to perform decompositions via compute(const MatrixType&, bool)
	* or compute(const MatrixType&, const MatrixType&, bool). The \p size
	* parameter is only used as a hint. It is not an error to give a wrong
	* \p size, but it may impair performance.
	*
	* \sa compute(const MatrixType&, bool) for an example
	*/
	SelfAdjointEigenSolver(Index size)
	: m_eivec(size, size),
	m_eivalues(size),
	m_subdiag(size > 1 ? size - 1 : 1),
	m_isInitialized(false)
	{}

	/** \brief Constructor; computes eigendecomposition of given matrix.
	*
	* \param[in] matrix Selfadjoint matrix whose eigendecomposition is to
	* be computed. Only the lower triangular part of the matrix is referenced.
	* \param[in] options Can be ComputeEigenvectors (default) or EigenvaluesOnly.
	*
	* This constructor calls compute(const MatrixType&, bool) to compute the
	* eigenvalues of the matrix \p matrix. The eigenvectors are computed if
	* \p options equals ComputeEigenvectors.
	*
	* Example: \include SelfAdjointEigenSolver_SelfAdjointEigenSolver_MatrixType.cpp
	* Output: \verbinclude SelfAdjointEigenSolver_SelfAdjointEigenSolver_MatrixType.out
	*
	* \sa compute(const MatrixType&, bool),
	* SelfAdjointEigenSolver(const MatrixType&, const MatrixType&, bool)
	*/
	SelfAdjointEigenSolver(const MatrixType& matrix, int options = ComputeEigenvectors)
	: m_eivec(matrix.rows(), matrix.cols()),
	m_eivalues(matrix.cols()),
	m_subdiag(matrix.rows() > 1 ? matrix.rows() - 1 : 1),
	m_isInitialized(false)
	{
	compute(matrix, options);
	}

	/** \brief Computes eigendecomposition of given matrix.
	*
	* \param[in] matrix Selfadjoint matrix whose eigendecomposition is to
	* be computed. Only the lower triangular part of the matrix is referenced.
	* \param[in] options Can be ComputeEigenvectors (default) or EigenvaluesOnly.
	* \returns Reference to \c *this
	*
	* This function computes the eigenvalues of \p matrix. The eigenvalues()
	* function can be used to retrieve them. If \p options equals ComputeEigenvectors,
	* then the eigenvectors are also computed and can be retrieved by
	* calling eigenvectors().
	*
	* This implementation uses a symmetric QR algorithm. The matrix is first
	* reduced to tridiagonal form using the Tridiagonalization class. The
	* tridiagonal matrix is then brought to diagonal form with implicit
	* symmetric QR steps with Wilkinson shift. Details can be found in
	* Section 8.3 of Golub \& Van Loan, <i>%Matrix Computations</i>.
	*
	* The cost of the computation is about \f$ 9n^3 \f$ if the eigenvectors
	* are required and \f$ 4n^3/3 \f$ if they are not required.
	*
	* This method reuses the memory in the SelfAdjointEigenSolver object that
	* was allocated when the object was constructed, if the size of the
	* matrix does not change.
	*
	* Example: \include SelfAdjointEigenSolver_compute_MatrixType.cpp
	* Output: \verbinclude SelfAdjointEigenSolver_compute_MatrixType.out
	*
	* \sa SelfAdjointEigenSolver(const MatrixType&, bool)
	*/
	SelfAdjointEigenSolver& compute(const MatrixType& matrix, int options = ComputeEigenvectors);

	/** \brief Returns the eigenvectors of given matrix (pencil).
	*
	* \returns A const reference to the matrix whose columns are the eigenvectors.
	*
	* \pre The eigenvectors have been computed before.
	*
	* Column \f$ k \f$ of the returned matrix is an eigenvector corresponding
	* to eigenvalue number \f$ k \f$ as returned by eigenvalues(). The
	* eigenvectors are normalized to have (Euclidean) norm equal to one. If
	* this object was used to solve the eigenproblem for the selfadjoint
	* matrix \f$ A \f$, then the matrix returned by this function is the
	* matrix \f$ V \f$ in the eigendecomposition \f$ A = V D V^{-1} \f$.
	*
	* Example: \include SelfAdjointEigenSolver_eigenvectors.cpp
	* Output: \verbinclude SelfAdjointEigenSolver_eigenvectors.out
	*
	* \sa eigenvalues()
	*/
	const MatrixType& eigenvectors() const
	{
	ei_assert(m_isInitialized && "SelfAdjointEigenSolver is not initialized.");
	ei_assert(m_eigenvectorsOk && "The eigenvectors have not been computed together with the eigenvalues.");
	return m_eivec;
	}

	/** \brief Returns the eigenvalues of given matrix (pencil).
	*
	* \returns A const reference to the column vector containing the eigenvalues.
	*
	* \pre The eigenvalues have been computed before.
	*
	* The eigenvalues are repeated according to their algebraic multiplicity,
	* so there are as many eigenvalues as rows in the matrix.
	*
	* Example: \include SelfAdjointEigenSolver_eigenvalues.cpp
	* Output: \verbinclude SelfAdjointEigenSolver_eigenvalues.out
	*
	* \sa eigenvectors(), MatrixBase::eigenvalues()
	*/
	const RealVectorType& eigenvalues() const
	{
	ei_assert(m_isInitialized && "SelfAdjointEigenSolver is not initialized.");
	return m_eivalues;
	}

	/** \brief Computes the positive-definite square root of the matrix.
	*
	* \returns the positive-definite square root of the matrix
	*
	* \pre The eigenvalues and eigenvectors of a positive-definite matrix
	* have been computed before.
	*
	* The square root of a positive-definite matrix \f$ A \f$ is the
	* positive-definite matrix whose square equals \f$ A \f$. This function
	* uses the eigendecomposition \f$ A = V D V^{-1} \f$ to compute the
	* square root as \f$ A^{1/2} = V D^{1/2} V^{-1} \f$.
	*
	* Example: \include SelfAdjointEigenSolver_operatorSqrt.cpp
	* Output: \verbinclude SelfAdjointEigenSolver_operatorSqrt.out
	*
	* \sa operatorInverseSqrt(),
	* \ref MatrixFunctions_Module "MatrixFunctions Module"
	*/
	MatrixType operatorSqrt() const
	{
	ei_assert(m_isInitialized && "SelfAdjointEigenSolver is not initialized.");
	ei_assert(m_eigenvectorsOk && "The eigenvectors have not been computed together with the eigenvalues.");
	return m_eivec * m_eivalues.cwiseSqrt().asDiagonal() * m_eivec.adjoint();
	}

	/** \brief Computes the inverse square root of the matrix.
	*
	* \returns the inverse positive-definite square root of the matrix
	*
	* \pre The eigenvalues and eigenvectors of a positive-definite matrix
	* have been computed before.
	*
	* This function uses the eigendecomposition \f$ A = V D V^{-1} \f$ to
	* compute the inverse square root as \f$ V D^{-1/2} V^{-1} \f$. This is
	* cheaper than first computing the square root with operatorSqrt() and
	* then its inverse with MatrixBase::inverse().
	*
	* Example: \include SelfAdjointEigenSolver_operatorInverseSqrt.cpp
	* Output: \verbinclude SelfAdjointEigenSolver_operatorInverseSqrt.out
	*
	* \sa operatorSqrt(), MatrixBase::inverse(),
	* \ref MatrixFunctions_Module "MatrixFunctions Module"
	*/
	MatrixType operatorInverseSqrt() const
	{
	ei_assert(m_isInitialized && "SelfAdjointEigenSolver is not initialized.");
	ei_assert(m_eigenvectorsOk && "The eigenvectors have not been computed together with the eigenvalues.");
	return m_eivec * m_eivalues.cwiseInverse().cwiseSqrt().asDiagonal() * m_eivec.adjoint();
	}

	/** \brief Reports whether previous computation was successful.
	*
	* \returns \c Success if computation was succesful, \c NoConvergence otherwise.
	*/
	ComputationInfo info() const
	{
	ei_assert(m_isInitialized && "SelfAdjointEigenSolver is not initialized.");
	return m_info;
	}

	/** \brief Maximum number of iterations.
	*
	* Maximum number of iterations allowed for an eigenvalue to converge.
	*/
	static const int m_maxIterations = 30;

	protected:
	MatrixType m_eivec;
	RealVectorType m_eivalues;
	typename TridiagonalizationType::SubDiagonalType m_subdiag;
	ComputationInfo m_info;
	bool m_isInitialized;
	bool m_eigenvectorsOk;
	};

	/** \internal
	*
	* \eigenvalues_module \ingroup Eigenvalues_Module
	*
	* Performs a QR step on a tridiagonal symmetric matrix represented as a
	* pair of two vectors \a diag and \a subdiag.
	*
	* \param matA the input selfadjoint matrix
	* \param hCoeffs returned Householder coefficients
	*
	* For compilation efficiency reasons, this procedure does not use eigen expression
	* for its arguments.
	*
	* Implemented from Golub's "Matrix Computations", algorithm 8.3.2:
	* "implicit symmetric QR step with Wilkinson shift"
	*/
	template<typename RealScalar, typename Scalar, typename Index>
	static void ei_tridiagonal_qr_step(RealScalar* diag, RealScalar* subdiag, Index start, Index end, Scalar* matrixQ, Index n);

	template<typename MatrixType>
	SelfAdjointEigenSolver<MatrixType>& SelfAdjointEigenSolver<MatrixType>
	::compute(const MatrixType& matrix, int options)
	{
	ei_assert(matrix.cols() == matrix.rows());
	ei_assert((options&~(EigVecMask\|GenEigMask))==0
	&& (options&EigVecMask)!=EigVecMask
	&& "invalid option parameter");
	bool computeEigenvectors = (options&ComputeEigenvectors)==ComputeEigenvectors;
	Index n = matrix.cols();
	m_eivalues.resize(n,1);

	if(n==1)
	{
	m_eivalues.coeffRef(0,0) = ei_real(matrix.coeff(0,0));
	if(computeEigenvectors)
	m_eivec.setOnes();
	m_info = Success;
	m_isInitialized = true;
	m_eigenvectorsOk = computeEigenvectors;
	return *this;
	}

	// declare some aliases
	RealVectorType& diag = m_eivalues;
	MatrixType& mat = m_eivec;

	mat = matrix;
	m_subdiag.resize(n-1);
	ei_tridiagonalization_inplace(mat, diag, m_subdiag, computeEigenvectors);

	Index end = n-1;
	Index start = 0;
	Index iter = 0; // number of iterations we are working on one element

	while (end>0)
	{
	for (Index i = start; i<end; ++i)
	if (ei_isMuchSmallerThan(ei_abs(m_subdiag[i]),(ei_abs(diag[i])+ei_abs(diag[i+1]))))
	m_subdiag[i] = 0;

	// find the largest unreduced block
	while (end>0 && m_subdiag[end-1]==0)
	{
	iter = 0;
	end--;
	}
	if (end<=0)
	break;

	// if we spent too many iterations on the current element, we give up
	iter++;
	if(iter > m_maxIterations) break;

	start = end - 1;
	while (start>0 && m_subdiag[start-1]!=0)
	start--;

	ei_tridiagonal_qr_step(diag.data(), m_subdiag.data(), start, end, computeEigenvectors ? m_eivec.data() : (Scalar*)0, n);
	}

	if (iter <= m_maxIterations)
	m_info = Success;
	else
	m_info = NoConvergence;

	// Sort eigenvalues and corresponding vectors.
	// TODO make the sort optional ?
	// TODO use a better sort algorithm !!
	if (m_info == Success)
	{
	for (Index i = 0; i < n-1; ++i)
	{
	Index k;
	m_eivalues.segment(i,n-i).minCoeff(&k);
	if (k > 0)
	{
	std::swap(m_eivalues[i], m_eivalues[k+i]);
	if(computeEigenvectors)
	m_eivec.col(i).swap(m_eivec.col(k+i));
	}
	}
	}

	m_isInitialized = true;
	m_eigenvectorsOk = computeEigenvectors;
	return *this;
	}

	template<typename RealScalar, typename Scalar, typename Index>
	static void ei_tridiagonal_qr_step(RealScalar* diag, RealScalar* subdiag, Index start, Index end, Scalar* matrixQ, Index n)
	{
	RealScalar td = (diag[end-1] - diag[end])*RealScalar(0.5);
	RealScalar e2 = ei_abs2(subdiag[end-1]);
	RealScalar mu = diag[end] - e2 / (td + (td>0 ? 1 : -1) * ei_sqrt(td*td + e2));
	RealScalar x = diag[start] - mu;
	RealScalar z = subdiag[start];

	for (Index k = start; k < end; ++k)
	{
	PlanarRotation<RealScalar> rot;
	rot.makeGivens(x, z);

	// do T = G' T G
	RealScalar sdk = rot.s() * diag[k] + rot.c() * subdiag[k];
	RealScalar dkp1 = rot.s() * subdiag[k] + rot.c() * diag[k+1];

	diag[k] = rot.c() * (rot.c() * diag[k] - rot.s() * subdiag[k]) - rot.s() * (rot.c() * subdiag[k] - rot.s() * diag[k+1]);
	diag[k+1] = rot.s() * sdk + rot.c() * dkp1;
	subdiag[k] = rot.c() * sdk - rot.s() * dkp1;

	if (k > start)
	subdiag[k - 1] = rot.c() * subdiag[k-1] - rot.s() * z;

	x = subdiag[k];

	if (k < end - 1)
	{
	z = -rot.s() * subdiag[k+1];
	subdiag[k + 1] = rot.c() * subdiag[k+1];
	}

	// apply the givens rotation to the unit matrix Q = Q * G
	if (matrixQ)
	{
	Map<Matrix<Scalar,Dynamic,Dynamic> > q(matrixQ,n,n);
	q.applyOnTheRight(k,k+1,rot);
	}
	}
	}

	#endif // EIGEN_SELFADJOINTEIGENSOLVER_H