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DSBEVX(3F)							    DSBEVX(3F)


NAME    [Toc]    [Back]

     DSBEVX - compute selected eigenvalues and,	optionally, eigenvectors of a
     real symmetric band matrix	A

SYNOPSIS    [Toc]    [Back]

     SUBROUTINE	DSBEVX(	JOBZ, RANGE, UPLO, N, KD, AB, LDAB, Q, LDQ, VL,	VU,
			IL, IU,	ABSTOL,	M, W, Z, LDZ, WORK, IWORK, IFAIL, INFO
			)

	 CHARACTER	JOBZ, RANGE, UPLO

	 INTEGER	IL, INFO, IU, KD, LDAB,	LDQ, LDZ, M, N

	 DOUBLE		PRECISION ABSTOL, VL, VU

	 INTEGER	IFAIL( * ), IWORK( * )

	 DOUBLE		PRECISION AB( LDAB, * ), Q( LDQ, * ), W( * ), WORK( *
			), Z( LDZ, * )

PURPOSE    [Toc]    [Back]

     DSBEVX computes selected eigenvalues and, optionally, eigenvectors	of a
     real symmetric band matrix	A.  Eigenvalues	and eigenvectors can be
     selected by specifying either a range of values or	a range	of indices for
     the desired eigenvalues.

ARGUMENTS    [Toc]    [Back]

     JOBZ    (input) CHARACTER*1
	     = 'N':  Compute eigenvalues only;
	     = 'V':  Compute eigenvalues and eigenvectors.

     RANGE   (input) CHARACTER*1
	     = 'A': all	eigenvalues will be found;
	     = 'V': all	eigenvalues in the half-open interval (VL,VU] will be
	     found; = 'I': the IL-th through IU-th eigenvalues will be found.

     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.

     KD	     (input) INTEGER
	     The number	of superdiagonals of the matrix	A if UPLO = 'U', or
	     the number	of subdiagonals	if UPLO	= 'L'.	KD >= 0.

     AB	     (input/output) DOUBLE PRECISION array, dimension (LDAB, N)
	     On	entry, the upper or lower triangle of the symmetric band
	     matrix A, stored in the first KD+1	rows of	the array.  The	j-th
	     column of A is stored in the j-th column of the array AB as



									Page 1






DSBEVX(3F)							    DSBEVX(3F)



	     follows:  if UPLO = 'U', AB(kd+1+i-j,j) = A(i,j) for max(1,jkd)<=i<=j;
	if UPLO	= 'L', AB(1+i-j,j)    =	A(i,j) for
	     j<=i<=min(n,j+kd).

	     On	exit, AB is overwritten	by values generated during the
	     reduction to tridiagonal form.  If	UPLO = 'U', the	first
	     superdiagonal and the diagonal of the tridiagonal matrix T	are
	     returned in rows KD and KD+1 of AB, and if	UPLO = 'L', the
	     diagonal and first	subdiagonal of T are returned in the first two
	     rows of AB.

     LDAB    (input) INTEGER
	     The leading dimension of the array	AB.  LDAB >= KD	+ 1.

     Q	     (output) DOUBLE PRECISION array, dimension	(LDQ, N)
	     If	JOBZ = 'V', the	N-by-N orthogonal matrix used in the reduction
	     to	tridiagonal form.  If JOBZ = 'N', the array Q is not
	     referenced.

     LDQ     (input) INTEGER
	     The leading dimension of the array	Q.  If JOBZ = 'V', then	LDQ >=
	     max(1,N).

     VL	     (input) DOUBLE PRECISION
	     VU	     (input) DOUBLE PRECISION If RANGE='V', the	lower and
	     upper bounds of the interval to be	searched for eigenvalues. VL <
	     VU.  Not referenced if RANGE = 'A'	or 'I'.

     IL	     (input) INTEGER
	     IU	     (input) INTEGER If	RANGE='I', the indices (in ascending
	     order) of the smallest and	largest	eigenvalues to be returned.  1
	     <=	IL <= IU <= N, if N > 0; IL = 1	and IU = 0 if N	= 0.  Not
	     referenced	if RANGE = 'A' or 'V'.

     ABSTOL  (input) DOUBLE PRECISION
	     The absolute error	tolerance for the eigenvalues.	An approximate
	     eigenvalue	is accepted as converged when it is determined to lie
	     in	an interval [a,b] of width less	than or	equal to

	     ABSTOL + EPS *   max( |a|,|b| ) ,

	     where EPS is the machine precision.  If ABSTOL is less than or
	     equal to zero, then  EPS*|T|  will	be used	in its place, where
	     |T| is the	1-norm of the tridiagonal matrix obtained by reducing
	     AB	to tridiagonal form.

	     Eigenvalues will be computed most accurately when ABSTOL is set
	     to	twice the underflow threshold 2*DLAMCH('S'), not zero.	If
	     this routine returns with INFO>0, indicating that some
	     eigenvectors did not converge, try	setting	ABSTOL to
	     2*DLAMCH('S').




									Page 2






DSBEVX(3F)							    DSBEVX(3F)



	     See "Computing Small Singular Values of Bidiagonal	Matrices with
	     Guaranteed	High Relative Accuracy," by Demmel and Kahan, LAPACK
	     Working Note #3.

     M	     (output) INTEGER
	     The total number of eigenvalues found.  0 <= M <= N.  If RANGE =
	     'A', M = N, and if	RANGE =	'I', M = IU-IL+1.

     W	     (output) DOUBLE PRECISION array, dimension	(N)
	     The first M elements contain the selected eigenvalues in
	     ascending order.

     Z	     (output) DOUBLE PRECISION array, dimension	(LDZ, max(1,M))
	     If	JOBZ = 'V', then if INFO = 0, the first	M columns of Z contain
	     the orthonormal eigenvectors of the matrix	A corresponding	to the
	     selected eigenvalues, with	the i-th column	of Z holding the
	     eigenvector associated with W(i).	If an eigenvector fails	to
	     converge, then that column	of Z contains the latest approximation
	     to	the eigenvector, and the index of the eigenvector is returned
	     in	IFAIL.	If JOBZ	= 'N', then Z is not referenced.  Note:	the
	     user must ensure that at least max(1,M) columns are supplied in
	     the array Z; if RANGE = 'V', the exact value of M is not known in
	     advance and an upper bound	must be	used.

     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 (7*N)

     IWORK   (workspace) INTEGER array,	dimension (5*N)

     IFAIL   (output) INTEGER array, dimension (N)
	     If	JOBZ = 'V', then if INFO = 0, the first	M elements of IFAIL
	     are zero.	If INFO	> 0, then IFAIL	contains the indices of	the
	     eigenvectors that failed to converge.  If JOBZ = 'N', then	IFAIL
	     is	not referenced.

     INFO    (output) INTEGER
	     = 0:  successful exit.
	     < 0:  if INFO = -i, the i-th argument had an illegal value.
	     > 0:  if INFO = i,	then i eigenvectors failed to converge.	 Their
	     indices are stored	in array IFAIL.
DSBEVX(3F)							    DSBEVX(3F)


NAME    [Toc]    [Back]

     DSBEVX - compute selected eigenvalues and,	optionally, eigenvectors of a
     real symmetric band matrix	A

SYNOPSIS    [Toc]    [Back]

     SUBROUTINE	DSBEVX(	JOBZ, RANGE, UPLO, N, KD, AB, LDAB, Q, LDQ, VL,	VU,
			IL, IU,	ABSTOL,	M, W, Z, LDZ, WORK, IWORK, IFAIL, INFO
			)

	 CHARACTER	JOBZ, RANGE, UPLO

	 INTEGER	IL, INFO, IU, KD, LDAB,	LDQ, LDZ, M, N

	 DOUBLE		PRECISION ABSTOL, VL, VU

	 INTEGER	IFAIL( * ), IWORK( * )

	 DOUBLE		PRECISION AB( LDAB, * ), Q( LDQ, * ), W( * ), WORK( *
			), Z( LDZ, * )

PURPOSE    [Toc]    [Back]

     DSBEVX computes selected eigenvalues and, optionally, eigenvectors	of a
     real symmetric band matrix	A.  Eigenvalues	and eigenvectors can be
     selected by specifying either a range of values or	a range	of indices for
     the desired eigenvalues.

ARGUMENTS    [Toc]    [Back]

     JOBZ    (input) CHARACTER*1
	     = 'N':  Compute eigenvalues only;
	     = 'V':  Compute eigenvalues and eigenvectors.

     RANGE   (input) CHARACTER*1
	     = 'A': all	eigenvalues will be found;
	     = 'V': all	eigenvalues in the half-open interval (VL,VU] will be
	     found; = 'I': the IL-th through IU-th eigenvalues will be found.

     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.

     KD	     (input) INTEGER
	     The number	of superdiagonals of the matrix	A if UPLO = 'U', or
	     the number	of subdiagonals	if UPLO	= 'L'.	KD >= 0.

     AB	     (input/output) DOUBLE PRECISION array, dimension (LDAB, N)
	     On	entry, the upper or lower triangle of the symmetric band
	     matrix A, stored in the first KD+1	rows of	the array.  The	j-th
	     column of A is stored in the j-th column of the array AB as



									Page 1






DSBEVX(3F)							    DSBEVX(3F)



	     follows:  if UPLO = 'U', AB(kd+1+i-j,j) = A(i,j) for max(1,jkd)<=i<=j;
	if UPLO	= 'L', AB(1+i-j,j)    =	A(i,j) for
	     j<=i<=min(n,j+kd).

	     On	exit, AB is overwritten	by values generated during the
	     reduction to tridiagonal form.  If	UPLO = 'U', the	first
	     superdiagonal and the diagonal of the tridiagonal matrix T	are
	     returned in rows KD and KD+1 of AB, and if	UPLO = 'L', the
	     diagonal and first	subdiagonal of T are returned in the first two
	     rows of AB.

     LDAB    (input) INTEGER
	     The leading dimension of the array	AB.  LDAB >= KD	+ 1.

     Q	     (output) DOUBLE PRECISION array, dimension	(LDQ, N)
	     If	JOBZ = 'V', the	N-by-N orthogonal matrix used in the reduction
	     to	tridiagonal form.  If JOBZ = 'N', the array Q is not
	     referenced.

     LDQ     (input) INTEGER
	     The leading dimension of the array	Q.  If JOBZ = 'V', then	LDQ >=
	     max(1,N).

     VL	     (input) DOUBLE PRECISION
	     VU	     (input) DOUBLE PRECISION If RANGE='V', the	lower and
	     upper bounds of the interval to be	searched for eigenvalues. VL <
	     VU.  Not referenced if RANGE = 'A'	or 'I'.

     IL	     (input) INTEGER
	     IU	     (input) INTEGER If	RANGE='I', the indices (in ascending
	     order) of the smallest and	largest	eigenvalues to be returned.  1
	     <=	IL <= IU <= N, if N > 0; IL = 1	and IU = 0 if N	= 0.  Not
	     referenced	if RANGE = 'A' or 'V'.

     ABSTOL  (input) DOUBLE PRECISION
	     The absolute error	tolerance for the eigenvalues.	An approximate
	     eigenvalue	is accepted as converged when it is determined to lie
	     in	an interval [a,b] of width less	than or	equal to

	     ABSTOL + EPS *   max( |a|,|b| ) ,

	     where EPS is the machine precision.  If ABSTOL is less than or
	     equal to zero, then  EPS*|T|  will	be used	in its place, where
	     |T| is the	1-norm of the tridiagonal matrix obtained by reducing
	     AB	to tridiagonal form.

	     Eigenvalues will be computed most accurately when ABSTOL is set
	     to	twice the underflow threshold 2*DLAMCH('S'), not zero.	If
	     this routine returns with INFO>0, indicating that some
	     eigenvectors did not converge, try	setting	ABSTOL to
	     2*DLAMCH('S').




									Page 2






DSBEVX(3F)							    DSBEVX(3F)



	     See "Computing Small Singular Values of Bidiagonal	Matrices with
	     Guaranteed	High Relative Accuracy," by Demmel and Kahan, LAPACK
	     Working Note #3.

     M	     (output) INTEGER
	     The total number of eigenvalues found.  0 <= M <= N.  If RANGE =
	     'A', M = N, and if	RANGE =	'I', M = IU-IL+1.

     W	     (output) DOUBLE PRECISION array, dimension	(N)
	     The first M elements contain the selected eigenvalues in
	     ascending order.

     Z	     (output) DOUBLE PRECISION array, dimension	(LDZ, max(1,M))
	     If	JOBZ = 'V', then if INFO = 0, the first	M columns of Z contain
	     the orthonormal eigenvectors of the matrix	A corresponding	to the
	     selected eigenvalues, with	the i-th column	of Z holding the
	     eigenvector associated with W(i).	If an eigenvector fails	to
	     converge, then that column	of Z contains the latest approximation
	     to	the eigenvector, and the index of the eigenvector is returned
	     in	IFAIL.	If JOBZ	= 'N', then Z is not referenced.  Note:	the
	     user must ensure that at least max(1,M) columns are supplied in
	     the array Z; if RANGE = 'V', the exact value of M is not known in
	     advance and an upper bound	must be	used.

     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 (7*N)

     IWORK   (workspace) INTEGER array,	dimension (5*N)

     IFAIL   (output) INTEGER array, dimension (N)
	     If	JOBZ = 'V', then if INFO = 0, the first	M elements of IFAIL
	     are zero.	If INFO	> 0, then IFAIL	contains the indices of	the
	     eigenvectors that failed to converge.  If JOBZ = 'N', then	IFAIL
	     is	not referenced.

     INFO    (output) INTEGER
	     = 0:  successful exit.
	     < 0:  if INFO = -i, the i-th argument had an illegal value.
	     > 0:  if INFO = i,	then i eigenvectors failed to converge.	 Their
	     indices are stored	in array IFAIL.


									PPPPaaaaggggeeee 3333
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