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SSYEVX Example Program in Fortran
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* =============================================================================
*
* SSYEVX Example.
* ==============
*
* Program computes the smallest eigenvalues and the corresponding
* eigenvectors of a real symmetric matrix A:
*
* 6.29 -0.39 0.61 1.18 -0.08
* -0.39 7.19 0.81 1.19 -0.08
* 0.61 0.81 5.48 -3.13 0.22
* 1.18 1.19 -3.13 3.79 -0.26
* -0.08 -0.08 0.22 -0.26 0.83
*
* Description.
* ============
*
* The routine computes selected eigenvalues and, optionally, eigenvectors of
* an n-by-n real symmetric matrix A. The eigenvector v(j) of A satisfies
*
* A*v(j) = lambda(j)*v(j)
*
* where lambda(j) is its eigenvalue. The computed eigenvectors are
* orthonormal.
* Eigenvalues and eigenvectors can be selected by specifying either a range
* of values or a range of indices for the desired eigenvalues.
*
* Example Program Results.
* ========================
*
* SSYEVX Example Program Results
*
* The total number of eigenvalues found: 3
*
* Selected eigenvalues
* 0.71 0.82 6.58
*
* Selected eigenvectors (stored columnwise)
* 0.22 0.09 -0.95
* 0.21 0.08 -0.04
* -0.52 -0.22 -0.29
* -0.73 -0.21 -0.09
* -0.32 0.94 0.01
* =============================================================================
*
* .. Parameters ..
INTEGER N, NSELECT
PARAMETER ( N = 5, NSELECT = 3 )
INTEGER LDA, LDZ
PARAMETER ( LDA = N, LDZ = N )
INTEGER LWMAX
PARAMETER ( LWMAX = 1000 )
*
* .. Local Scalars ..
INTEGER INFO, LWORK, IL, IU, M
REAL ABSTOL, VL, VU
*
* .. Local Arrays ..
* IWORK dimension should be at least 5*N
INTEGER IFAIL( N ), IWORK( 5*N )
REAL A( LDA, N ), W( N ), Z( LDZ, NSELECT ),
$ WORK( LWMAX )
DATA A/
$ 6.29, 0.00, 0.00, 0.00, 0.00,
$ -0.39, 7.19, 0.00, 0.00, 0.00,
$ 0.61, 0.81, 5.48, 0.00, 0.00,
$ 1.18, 1.19,-3.13, 3.79, 0.00,
$ -0.08,-0.08, 0.22,-0.26, 0.83
$ /
*
* .. External Subroutines ..
EXTERNAL SSYEVX
EXTERNAL PRINT_MATRIX
*
* .. Intrinsic Functions ..
INTRINSIC INT, MIN
*
* .. Executable Statements ..
WRITE(*,*)'SSYEVX Example Program Results'
* Negative ABSTOL means using the default value
ABSTOL = -1.0
* Set IL, IU to compute NSELECT smallest eigenvalues
IL = 1
IU = NSELECT
*
* Query the optimal workspace.
*
LWORK = -1
CALL SSYEVX( 'Vectors', 'Indices', 'Upper', N, A, LDA, VL, VU, IL,
$ IU, ABSTOL, M, W, Z, LDZ, WORK, LWORK, IWORK, IFAIL,
$ INFO )
LWORK = MIN( LWMAX, INT( WORK( 1 ) ) )
*
* Solve eigenproblem.
*
CALL SSYEVX( 'Vectors', 'Indices', 'Upper', N, A, LDA, VL, VU, IL,
$ IU, ABSTOL, M, W, Z, LDZ, WORK, LWORK, IWORK, IFAIL,
$ INFO )
*
* Check for convergence.
*
IF( INFO.GT.0 ) THEN
WRITE(*,*)'The algorithm failed to compute eigenvalues.'
STOP
END IF
*
* Print the number of eigenvalues found.
*
WRITE(*,'(/A,I2)')' The total number of eigenvalues found:', M
*
* Print eigenvalues.
*
CALL PRINT_MATRIX( 'Selected eigenvalues', 1, M, W, 1 )
*
* Print eigenvectors.
*
CALL PRINT_MATRIX( 'Selected eigenvectors (stored columnwise)',
$ N, M, Z, LDZ )
STOP
END
*
* End of SSYEVX Example.
*
* =============================================================================
*
* Auxiliary routine: printing a matrix.
*
SUBROUTINE PRINT_MATRIX( DESC, M, N, A, LDA )
CHARACTER*(*) DESC
INTEGER M, N, LDA
REAL A( LDA, * )
*
INTEGER I, J
*
WRITE(*,*)
WRITE(*,*) DESC
DO I = 1, M
WRITE(*,9998) ( A( I, J ), J = 1, N )
END DO
*
9998 FORMAT( 11(:,1X,F6.2) )
RETURN
END
Parent topic: SSYEVX Example