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+ SUBROUTINE DSPEV( JOBZ, UPLO, N, AP, W, Z, LDZ, WORK, INFO )
+*
+* -- LAPACK driver routine (version 3.1) --
+* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
+* November 2006
+*
+* .. Scalar Arguments ..
+ CHARACTER JOBZ, UPLO
+ INTEGER INFO, LDZ, N
+* ..
+* .. Array Arguments ..
+ DOUBLE PRECISION AP( * ), W( * ), WORK( * ), Z( LDZ, * )
+* ..
+*
+* Purpose
+* =======
+*
+* DSPEV computes all the eigenvalues and, optionally, eigenvectors of a
+* real symmetric matrix A in packed storage.
+*
+* Arguments
+* =========
+*
+* JOBZ (input) CHARACTER*1
+* = 'N': Compute eigenvalues only;
+* = 'V': Compute eigenvalues and eigenvectors.
+*
+* UPLO (input) CHARACTER*1
+* = 'U': Upper triangle of A is stored;
+* = 'L': Lower triangle of A is stored.
+*
+* N (input) INTEGER
+* The order of the matrix A. N >= 0.
+*
+* AP (input/output) DOUBLE PRECISION array, dimension (N*(N+1)/2)
+* On entry, the upper or lower triangle of the symmetric matrix
+* A, packed columnwise in a linear array. The j-th column of A
+* is stored in the array AP as follows:
+* if UPLO = 'U', AP(i + (j-1)*j/2) = A(i,j) for 1<=i<=j;
+* if UPLO = 'L', AP(i + (j-1)*(2*n-j)/2) = A(i,j) for j<=i<=n.
+*
+* On exit, AP is overwritten by values generated during the
+* reduction to tridiagonal form. If UPLO = 'U', the diagonal
+* and first superdiagonal of the tridiagonal matrix T overwrite
+* the corresponding elements of A, and if UPLO = 'L', the
+* diagonal and first subdiagonal of T overwrite the
+* corresponding elements of A.
+*
+* W (output) DOUBLE PRECISION array, dimension (N)
+* If INFO = 0, the eigenvalues in ascending order.
+*
+* Z (output) DOUBLE PRECISION array, dimension (LDZ, N)
+* If JOBZ = 'V', then if INFO = 0, Z contains the orthonormal
+* eigenvectors of the matrix A, with the i-th column of Z
+* holding the eigenvector associated with W(i).
+* If JOBZ = 'N', then Z is not referenced.
+*
+* LDZ (input) INTEGER
+* The leading dimension of the array Z. LDZ >= 1, and if
+* JOBZ = 'V', LDZ >= max(1,N).
+*
+* WORK (workspace) DOUBLE PRECISION array, dimension (3*N)
+*
+* INFO (output) INTEGER
+* = 0: successful exit.
+* < 0: if INFO = -i, the i-th argument had an illegal value.
+* > 0: if INFO = i, the algorithm failed to converge; i
+* off-diagonal elements of an intermediate tridiagonal
+* form did not converge to zero.
+*
+* =====================================================================
+*
+* .. Parameters ..
+ DOUBLE PRECISION ZERO, ONE
+ PARAMETER ( ZERO = 0.0D0, ONE = 1.0D0 )
+* ..
+* .. Local Scalars ..
+ LOGICAL WANTZ
+ INTEGER IINFO, IMAX, INDE, INDTAU, INDWRK, ISCALE
+ DOUBLE PRECISION ANRM, BIGNUM, EPS, RMAX, RMIN, SAFMIN, SIGMA,
+ $ SMLNUM
+* ..
+* .. External Functions ..
+ LOGICAL LSAME
+ DOUBLE PRECISION DLAMCH, DLANSP
+ EXTERNAL LSAME, DLAMCH, DLANSP
+* ..
+* .. External Subroutines ..
+ EXTERNAL DOPGTR, DSCAL, DSPTRD, DSTEQR, DSTERF, XERBLA
+* ..
+* .. Intrinsic Functions ..
+ INTRINSIC SQRT
+* ..
+* .. Executable Statements ..
+*
+* Test the input parameters.
+*
+ WANTZ = LSAME( JOBZ, 'V' )
+*
+ INFO = 0
+ IF( .NOT.( WANTZ .OR. LSAME( JOBZ, 'N' ) ) ) THEN
+ INFO = -1
+ ELSE IF( .NOT.( LSAME( UPLO, 'U' ) .OR. LSAME( UPLO, 'L' ) ) )
+ $ THEN
+ INFO = -2
+ ELSE IF( N.LT.0 ) THEN
+ INFO = -3
+ ELSE IF( LDZ.LT.1 .OR. ( WANTZ .AND. LDZ.LT.N ) ) THEN
+ INFO = -7
+ END IF
+*
+ IF( INFO.NE.0 ) THEN
+ CALL XERBLA( 'DSPEV ', -INFO )
+ RETURN
+ END IF
+*
+* Quick return if possible
+*
+ IF( N.EQ.0 )
+ $ RETURN
+*
+ IF( N.EQ.1 ) THEN
+ W( 1 ) = AP( 1 )
+ IF( WANTZ )
+ $ Z( 1, 1 ) = ONE
+ RETURN
+ END IF
+*
+* Get machine constants.
+*
+ SAFMIN = DLAMCH( 'Safe minimum' )
+ EPS = DLAMCH( 'Precision' )
+ SMLNUM = SAFMIN / EPS
+ BIGNUM = ONE / SMLNUM
+ RMIN = SQRT( SMLNUM )
+ RMAX = SQRT( BIGNUM )
+*
+* Scale matrix to allowable range, if necessary.
+*
+ ANRM = DLANSP( 'M', UPLO, N, AP, WORK )
+ ISCALE = 0
+ IF( ANRM.GT.ZERO .AND. ANRM.LT.RMIN ) THEN
+ ISCALE = 1
+ SIGMA = RMIN / ANRM
+ ELSE IF( ANRM.GT.RMAX ) THEN
+ ISCALE = 1
+ SIGMA = RMAX / ANRM
+ END IF
+ IF( ISCALE.EQ.1 ) THEN
+ CALL DSCAL( ( N*( N+1 ) ) / 2, SIGMA, AP, 1 )
+ END IF
+*
+* Call DSPTRD to reduce symmetric packed matrix to tridiagonal form.
+*
+ INDE = 1
+ INDTAU = INDE + N
+ CALL DSPTRD( UPLO, N, AP, W, WORK( INDE ), WORK( INDTAU ), IINFO )
+*
+* For eigenvalues only, call DSTERF. For eigenvectors, first call
+* DOPGTR to generate the orthogonal matrix, then call DSTEQR.
+*
+ IF( .NOT.WANTZ ) THEN
+ CALL DSTERF( N, W, WORK( INDE ), INFO )
+ ELSE
+ INDWRK = INDTAU + N
+ CALL DOPGTR( UPLO, N, AP, WORK( INDTAU ), Z, LDZ,
+ $ WORK( INDWRK ), IINFO )
+ CALL DSTEQR( JOBZ, N, W, WORK( INDE ), Z, LDZ, WORK( INDTAU ),
+ $ INFO )
+ END IF
+*
+* If matrix was scaled, then rescale eigenvalues appropriately.
+*
+ IF( ISCALE.EQ.1 ) THEN
+ IF( INFO.EQ.0 ) THEN
+ IMAX = N
+ ELSE
+ IMAX = INFO - 1
+ END IF
+ CALL DSCAL( IMAX, ONE / SIGMA, W, 1 )
+ END IF
+*
+ RETURN
+*
+* End of DSPEV
+*
+ END