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SSYEV Example Program in Fortran
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* =============================================================================
*
* SSYEV Example.
* ==============
*
* Program computes all eigenvalues and eigenvectors of a real symmetric
* matrix A:
*
* 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
*
* Description.
* ============
*
* The routine computes all 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.
*
* Example Program Results.
* ========================
*
* SSYEV Example Program Results
*
* Eigenvalues
* -11.07 -6.23 0.86 8.87 16.09
*
* Eigenvectors (stored columnwise)
* -0.30 -0.61 0.40 -0.37 0.49
* -0.51 -0.29 -0.41 -0.36 -0.61
* -0.08 -0.38 -0.66 0.50 0.40
* 0.00 -0.45 0.46 0.62 -0.46
* -0.80 0.45 0.17 0.31 0.16
* =============================================================================
*
* .. Parameters ..
INTEGER N
PARAMETER ( N = 5 )
INTEGER LDA
PARAMETER ( LDA = N )
INTEGER LWMAX
PARAMETER ( LWMAX = 1000 )
*
* .. Local Scalars ..
INTEGER INFO, LWORK
*
* .. Local Arrays ..
REAL A( LDA, N ), W( N ), WORK( LWMAX )
DATA A/
$ 1.96, 0.00, 0.00, 0.00, 0.00,
$ -6.49, 3.80, 0.00, 0.00, 0.00,
$ -0.47,-6.39, 4.17, 0.00, 0.00,
$ -7.20, 1.50,-1.51, 5.70, 0.00,
$ -0.65,-6.34, 2.67, 1.80,-7.10
$ /
*
* .. External Subroutines ..
EXTERNAL SSYEV
EXTERNAL PRINT_MATRIX
*
* .. Intrinsic Functions ..
INTRINSIC INT, MIN
*
* .. Executable Statements ..
WRITE(*,*)'SSYEV Example Program Results'
*
* Query the optimal workspace.
*
LWORK = -1
CALL SSYEV( 'Vectors', 'Upper', N, A, LDA, W, WORK, LWORK, INFO )
LWORK = MIN( LWMAX, INT( WORK( 1 ) ) )
*
* Solve eigenproblem.
*
CALL SSYEV( 'Vectors', 'Upper', N, A, LDA, W, WORK, LWORK, INFO )
*
* Check for convergence.
*
IF( INFO.GT.0 ) THEN
WRITE(*,*)'The algorithm failed to compute eigenvalues.'
STOP
END IF
*
* Print eigenvalues.
*
CALL PRINT_MATRIX( 'Eigenvalues', 1, N, W, 1 )
*
* Print eigenvectors.
*
CALL PRINT_MATRIX( 'Eigenvectors (stored columnwise)', N, N, A,
$ LDA )
STOP
END
*
* End of SSYEV 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: SSYEV Example