GetSymEigenValues_

This subroutine calculates eigenvalues of a symmetric matrix.

• For n=2 it calls SymEigenValues2y2
• For n=3 it calls SymEigenValues3by3
• For n>=4 it calls SYEV routine from Lapack95

caution

In this routine matrix mat will be destroyed. If you want to preserve mat, then call GetSymEigenValues.

܀ Interface

INTERFACE
  MODULE SUBROUTINE GetSymEigenValues_(mat, eigenValues)
    REAL(DFP), INTENT(INOUT) :: mat(:, :)
    REAL(DFP), INTENT(OUT) :: eigenValues(:)
  END SUBROUTINE GetSymEigenValues_
END INTERFACE

️܀ See example

caution

Note this routine destroys the matrix at output.

Modules and classes

Usage

PROGRAM main
USE easifemBase

    REAL( DFP ), ALLOCATABLE :: mat0(:,:), mat(:,:), w(:), exact(:)
    REAL( DFP ), PARAMETER :: tol= 1.0E-2

!!! note "Test-1"

    mat =RESHAPE( &
      [   1.96, -6.49, -0.47, -7.20, -0.65, &
        & -6.49, 3.80, -6.39, 1.50, -6.34, &
        & -0.47,-6.39, 4.17, -1.51, 2.67, &
        & -7.20, 1.50,-1.51, 5.70, 1.80, &
        & -0.65,-6.34, 2.67, 1.80,-7.10 &
      ], [5,5])
    mat0=mat
    !!
    CALL Reallocate(w, SIZE(mat,1))
    CALL GetSymEigenValues_(mat=mat, eigenValues=w)
    !!
    exact = [-11.07, -6.23, 0.86, 8.87, 16.09]
    !!
    CALL OK( ALL(SOFTEQ(w, exact, tol)), "")
    !!
    CALL Display(MdEncode(ABS(mat0-mat)), "mat0-mat=")

caution

At output mat is changed, the difference is given below. mat0-mat=

0.79415	10.448	0.47892	7.5634	0.5773
0	1.5357	2.4282	1.5005	5.6309
0	0	1.599	7.8722	2.3714
0	0	0	0.85741	8.9405
0	0	0	0	0

END PROGRAM main

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