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DSYEVR(l)					)					DSYEVR(l)

       DSYEVR  -  compute  selected eigenvalues and, optionally, eigenvectors of a real symmetric
       matrix T





	   INTEGER	  ISUPPZ( * ), IWORK( * )

	   DOUBLE	  PRECISION A( LDA, * ), W( * ), WORK( * ), Z( LDZ, * )

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

       Whenever possible, DSYEVR calls DSTEGR to compute the
       eigenspectrum using Relatively Robust Representations.  DSTEGR computes eigenvalues by the
       dqds algorithm, while orthogonal eigenvectors are computed from various	"good"	L  D  L^T
       representations	(also  known as Relatively Robust Representations). Gram-Schmidt orthogo-
       nalization is avoided as far as possible. More specifically,  the  various  steps  of  the
       algorithm are as follows. For the i-th unreduced block of T,
	  (a) Compute T - sigma_i = L_i D_i L_i^T, such that L_i D_i L_i^T
	       is a relatively robust representation,
	  (b) Compute the eigenvalues, lambda_j, of L_i D_i L_i^T to high
	      relative accuracy by the dqds algorithm,
	  (c) If there is a cluster of close eigenvalues, "choose" sigma_i
	      close to the cluster, and go to step (a),
	  (d) Given the approximate eigenvalue lambda_j of L_i D_i L_i^T,
	      compute the corresponding eigenvector by forming a
	      rank-revealing twisted factorization.
       The desired accuracy of the output can be specified by the input parameter ABSTOL.

       For  more  details,  see  "A  new O(n^2) algorithm for the symmetric tridiagonal eigenval-
       ue/eigenvector problem", by Inderjit Dhillon, Computer Science Division	Technical  Report
       No. UCB//CSD-97-971, UC Berkeley, May 1997.

       Note 1 : DSYEVR calls DSTEGR when the full spectrum is requested on machines which conform
       to the ieee-754 floating point standard.  DSYEVR  calls	DSTEBZ	and  SSTEIN  on  non-ieee
       machines and
       when partial spectrum requests are made.

       Normal  execution  of  DSTEGR  may create NaNs and infinities and hence may abort due to a
       floating point exception in environments which do not handle NaNs and  infinities  in  the
       ieee standard default manner.

       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.

       A       (input/output) DOUBLE PRECISION array, dimension (LDA, N)
	       On entry, the symmetric matrix A.  If UPLO = 'U', the leading N-by-N upper  trian-
	       gular  part  of	A  contains the upper triangular part of the matrix A.	If UPLO =
	       'L', the leading N-by-N lower triangular part of A contains the	lower  triangular
	       part of the matrix A.  On exit, the lower triangle (if UPLO='L') or the upper tri-
	       angle (if UPLO='U') of A, including the diagonal, is destroyed.

       LDA     (input) INTEGER
	       The leading dimension of the array A.  LDA >= 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'.

	       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 A to tridiagonal form.

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

	       If  high  relative  accuracy is important, set ABSTOL to DLAMCH( 'Safe minimum' ).
	       Doing so will guarantee that eigenvalues are computed to  high  relative  accuracy
	       when  possible  in future releases.  The current code does not make any guarantees
	       about high relative accuracy, but furutre releases will. See J. Barlow and J. Dem-
	       mel,  "Computing  Accurate  Eigensystems  of Scaled Diagonally Dominant Matrices",
	       LAPACK Working Note #7, for a discussion of which matrices define their	eigenval-
	       ues to high relative accuracy.

       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  JOBZ  =  'N',
	       then  Z is not referenced.  Note: the user must ensure that at least max(1,M) col-
	       umns 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 >=

       ISUPPZ  (output) INTEGER array, dimension ( 2*max(1,M) )
	       The support of the eigenvectors in Z, i.e., the	indices  indicating  the  nonzero
	       elements  in  Z.  The i-th eigenvector is nonzero only in elements ISUPPZ( 2*i-1 )
	       through ISUPPZ( 2*i ).

       WORK    (workspace/output) DOUBLE PRECISION array, dimension (LWORK)
	       On exit, if INFO = 0, WORK(1) returns the optimal LWORK.

       LWORK   (input) INTEGER
	       The dimension of the array WORK.  LWORK >= max(1,26*N).	For  optimal  efficiency,
	       LWORK  >=  (NB+6)*N,  where  NB	is the max of the blocksize for DSYTRD and DORMTR
	       returned by ILAENV.

	       If LWORK = -1, then a workspace query is assumed; the routine only calculates  the
	       optimal	size of the WORK array, returns this value as the first entry of the WORK
	       array, and no error message related to LWORK is issued by XERBLA.

       IWORK   (workspace/output) INTEGER array, dimension (LIWORK)
	       On exit, if INFO = 0, IWORK(1) returns the optimal LWORK.

       LIWORK  (input) INTEGER
	       The dimension of the array IWORK.  LIWORK >= max(1,10*N).

	       If LIWORK = -1, then a workspace query is assumed; the routine only calculates the
	       optimal	size  of  the  IWORK  array, returns this value as the first entry of the
	       IWORK array, and no error message related to LIWORK is issued by XERBLA.

       INFO    (output) INTEGER
	       = 0:  successful exit
	       < 0:  if INFO = -i, the i-th argument had an illegal value
	       > 0:  Internal error

       Based on contributions by
	  Inderjit Dhillon, IBM Almaden, USA
	  Osni Marques, LBNL/NERSC, USA
	  Ken Stanley, Computer Science Division, University of
	    California at Berkeley, USA

LAPACK version 3.0			   15 June 2000 				DSYEVR(l)
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