PCHEEVX(l)						   LAPACK routine (version 1.5) 						PCHEEVX(l)

NAME

       PCHEEVX - compute selected eigenvalues and, optionally, eigenvectors of a complex hermitian matrix A by calling the recommended sequence of
       ScaLAPACK routines

SYNOPSIS

       SUBROUTINE PCHEEVX( JOBZ, RANGE, UPLO, N, A, IA, JA, DESCA, VL, VU, IL, IU, ABSTOL, M, NZ, W, ORFAC, Z, IZ, JZ, DESCZ, WORK, LWORK,  RWORK,
			   LRWORK, IWORK, LIWORK, IFAIL, ICLUSTR, GAP, INFO )

	   CHARACTER	   JOBZ, RANGE, UPLO

	   INTEGER	   IA, IL, INFO, IU, IZ, JA, JZ, LIWORK, LRWORK, LWORK, M, N, NZ

	   REAL 	   ABSTOL, ORFAC, VL, VU

	   INTEGER	   DESCA( * ), DESCZ( * ), ICLUSTR( * ), IFAIL( * ), IWORK( * )

	   REAL 	   GAP( * ), RWORK( * ), W( * )

	   COMPLEX	   A( * ), WORK( * ), Z( * )

	   INTEGER	   BLOCK_CYCLIC_2D, DLEN_, DTYPE_, CTXT_, M_, N_, MB_, NB_, RSRC_, CSRC_, LLD_

	   PARAMETER	   ( BLOCK_CYCLIC_2D = 1, DLEN_ = 9, DTYPE_ = 1, CTXT_ = 2, M_ = 3, N_ = 4, MB_ = 5, NB_ = 6, RSRC_ = 7, CSRC_ = 8, LLD_ =
			   9 )

	   REAL 	   ZERO, ONE, TEN, FIVE

	   PARAMETER	   ( ZERO = 0.0E+0, ONE = 1.0E+0, TEN = 10.0E+0, FIVE = 5.0E+0 )

	   INTEGER	   IERREIN, IERRCLS, IERRSPC, IERREBZ

	   PARAMETER	   ( IERREIN = 1, IERRCLS = 2, IERRSPC = 4, IERREBZ = 8 )

	   LOGICAL	   ALLEIG, INDEIG, LOWER, LQUERY, QUICKRETURN, VALEIG, WANTZ

	   CHARACTER	   ORDER

	   INTEGER	   CSRC_A, I, IACOL, IAROW, ICOFFA, IINFO, INDD, INDD2, INDE, INDE2, INDIBL, INDISP, INDRWORK,	INDTAU,  INDWORK,  IROFFA,
			   IROFFZ,  ISCALE,  ISIZESTEBZ, ISIZESTEIN, IZROW, LALLWORK, LIWMIN, LLRWORK, LLWORK, LRWMIN, LWMIN, MAXEIGS, MB_A, MB_Z,
			   MQ0, MYCOL, MYROW, NB, NB_A, NB_Z, NEIG, NN, NNP, NP0, NPCOL, NPROCS, NPROW, NQ0, NSPLIT, NZZ, OFFSET, RSRC_A,  RSRC_Z,
			   SIZEHEEVX, SIZEORMTR, SIZESTEIN

	   REAL 	   ABSTLL, ANRM, BIGNUM, EPS, RMAX, RMIN, SAFMIN, SIGMA, SMLNUM, VLL, VUU

	   INTEGER	   IDUM1( 4 ), IDUM2( 4 )

	   LOGICAL	   LSAME

	   INTEGER	   ICEIL, INDXG2P, NUMROC

	   REAL 	   PCLANHE, PSLAMCH

	   EXTERNAL	   LSAME, ICEIL, INDXG2P, NUMROC, PCLANHE, PSLAMCH

	   EXTERNAL	   BLACS_GRIDINFO,  CHK1MAT,  IGAMN2D, PCELGET, PCHETRD, PCHK2MAT, PCLASCL, PCSTEIN, PCUNMTR, PSLARED1D, PSSTEBZ, PXERBLA,
			   SGEBR2D, SGEBS2D, SLASRT, SSCAL

	   INTRINSIC	   ABS, CMPLX, ICHAR, MAX, MIN, MOD, REAL, SQRT

PURPOSE

       PCHEEVX computes selected eigenvalues and, optionally, eigenvectors of a complex hermitian matrix A by calling the recommended sequence	of
       ScaLAPACK routines.  Eigenvalues/vectors can be selected by specifying a range of values or a range of indices for the desired eigenvalues.

NOTES

       Each  global  data  object  is described by an associated description vector.  This vector stores the information required to establish the
       mapping between an object element and its corresponding process and memory location.

       Let A be a generic term for any 2D block cyclicly distributed array.  Such a global array has an associated description vector  DESCA.	In
       the following comments, the character _ should be read as "of the global array".

       NOTATION        STORED IN      EXPLANATION
       --------------- -------------- --------------------------------------
       DTYPE_A(global) DESCA( DTYPE_ )The descriptor type.  In this case,
				      DTYPE_A = 1.
       CTXT_A (global) DESCA( CTXT_ ) The BLACS context handle, indicating
				      the BLACS process grid A is distribu-
				      ted over. The context itself is glo-
				      bal, but the handle (the integer
				      value) may vary.
       M_A    (global) DESCA( M_ )    The number of rows in the global
				      array A.
       N_A    (global) DESCA( N_ )    The number of columns in the global
				      array A.
       MB_A   (global) DESCA( MB_ )   The blocking factor used to distribute
				      the rows of the array.
       NB_A   (global) DESCA( NB_ )   The blocking factor used to distribute
				      the columns of the array.
       RSRC_A (global) DESCA( RSRC_ ) The process row over which the first
				      row of the array A is distributed.
       CSRC_A (global) DESCA( CSRC_ ) The process column over which the
				      first column of the array A is
				      distributed.
       LLD_A  (local)  DESCA( LLD_ )  The leading dimension of the local
				      array.  LLD_A >= MAX(1,LOCr(M_A)).

       Let  K  be  the number of rows or columns of a distributed matrix, and assume that its process grid has dimension p x q.  LOCr( K ) denotes
       the number of elements of K that a process would receive if K were distributed over the p processes of its process column.S
       Similarly, LOCc( K ) denotes the number of elements of K that a process would receive if K were distributed over the  q	processes  of  its
       process row.
       The values of LOCr() and LOCc() may be determined via a call to the ScaLAPACK tool function, NUMROC:
	       LOCr( M ) = NUMROC( M, MB_A, MYROW, RSRC_A, NPROW ),
	       LOCc( N ) = NUMROC( N, NB_A, MYCOL, CSRC_A, NPCOL ).
       An upper bound for these quantities may be computed by:
	       LOCr( M ) <= ceil( ceil(M/MB_A)/NPROW )*MB_A
	       LOCc( N ) <= ceil( ceil(N/NB_A)/NPCOL )*NB_A

       PCHEEVX	assumes IEEE 754 standard compliant arithmetic.  To port to a system which does not have IEEE 754 arithmetic, modify the appropri-
       ate SLmake.inc file to include the compiler switch -DNO_IEEE.  This switch only affects the compilation of pslaiect.c.

ARGUMENTS

	  NP = the number of rows local to a given process.
	  NQ = the number of columns local to a given process.

       JOBZ    (global input) CHARACTER*1
	       Specifies whether or not to compute the eigenvectors:
	       = 'N':  Compute eigenvalues only.
	       = 'V':  Compute eigenvalues and eigenvectors.

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

       UPLO    (global input) CHARACTER*1
	       Specifies whether the upper or lower triangular part of the
	       Hermitian matrix A is stored:
	       = 'U':  Upper triangular
	       = 'L':  Lower triangular

       N       (global input) INTEGER
	       The number of rows and columns of the matrix A.	N >= 0.

       A       (local input/workspace) block cyclic COMPLEX array,
	       global dimension (N, N),
	       local dimension ( LLD_A, LOCc(JA+N-1) )

	       On entry, the Hermitian matrix A.  If UPLO = 'U', only the
	       upper triangular part of A is used to define the elements of
	       the Hermitian matrix.  If UPLO = 'L', only the lower
	       triangular part of A is used to define the elements of the
	       Hermitian matrix.

	       On exit, the lower triangle (if UPLO='L') or the upper
	       triangle (if UPLO='U') of A, including the diagonal, is
	       destroyed.

       IA      (global input) INTEGER
	       A's global row index, which points to the beginning of the
	       submatrix which is to be operated on.

       JA      (global input) INTEGER
	       A's global column index, which points to the beginning of
	       the submatrix which is to be operated on.

       DESCA   (global and local input) INTEGER array of dimension DLEN_.
	       The array descriptor for the distributed matrix A.
	       If DESCA( CTXT_ ) is incorrect, PCHEEVX cannot guarantee
	       correct error reporting.

       VL      (global input) REAL
	       If RANGE='V', the lower bound of the interval to be searched
	       for eigenvalues.  Not referenced if RANGE = 'A' or 'I'.

       VU      (global input) REAL
	       If RANGE='V', the upper bound of the interval to be searched
	       for eigenvalues.  Not referenced if RANGE = 'A' or 'I'.

       IL      (global input) INTEGER
	       If RANGE='I', the index (from smallest to largest) of the
	       smallest eigenvalue to be returned.  IL >= 1.
	       Not referenced if RANGE = 'A' or 'V'.

       IU      (global input) INTEGER
	       If RANGE='I', the index (from smallest to largest) of the
	       largest eigenvalue to be returned.  min(IL,N) <= IU <= N.
	       Not referenced if RANGE = 'A' or 'V'.

       ABSTOL  (global input) REAL
	       If JOBZ='V', setting ABSTOL to PSLAMCH( CONTEXT, 'U') yields
	       the most orthogonal eigenvectors.

	       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*norm(T) will be used in its place,
	       where norm(T) is the 1-norm of the tridiagonal matrix
	       obtained by reducing A to tridiagonal form.

	       Eigenvalues will be computed most accurately when ABSTOL is
	       set to twice the underflow threshold 2*PSLAMCH('S') not zero.
	       If this routine returns with ((MOD(INFO,2).NE.0) .OR.
	       (MOD(INFO/8,2).NE.0)), indicating that some eigenvalues or
	       eigenvectors did not converge, try setting ABSTOL to
	       2*PSLAMCH('S').

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

	       See "On the correctness of Parallel Bisection in Floating
	       Point" by Demmel, Dhillon and Ren, LAPACK Working Note #70

       M       (global output) INTEGER
	       Total number of eigenvalues found.  0 <= M <= N.

       NZ      (global output) INTEGER
	       Total number of eigenvectors computed.  0 <= NZ <= M.
	       The number of columns of Z that are filled.
	       If JOBZ .NE. 'V', NZ is not referenced.
	       If JOBZ .EQ. 'V', NZ = M unless the user supplies
	       insufficient space and PCHEEVX is not able to detect this
	       before beginning computation.  To get all the eigenvectors
	       requested, the user must supply both sufficient
	       space to hold the eigenvectors in Z (M .LE. DESCZ(N_))
	       and sufficient workspace to compute them.  (See LWORK below.)
	       PCHEEVX is always able to detect insufficient space without
	       computation unless RANGE .EQ. 'V'.

       W       (global output) REAL array, dimension (N)
	       On normal exit, the first M entries contain the selected
	       eigenvalues in ascending order.

       ORFAC   (global input) REAL
	       Specifies which eigenvectors should be reorthogonalized.
	       Eigenvectors that correspond to eigenvalues which are within
	       tol=ORFAC*norm(A) of each other are to be reorthogonalized.
	       However, if the workspace is insufficient (see LWORK),
	       tol may be decreased until all eigenvectors to be
	       reorthogonalized can be stored in one process.
	       No reorthogonalization will be done if ORFAC equals zero.
	       A default value of 10^-3 is used if ORFAC is negative.
	       ORFAC should be identical on all processes.

       Z       (local output) COMPLEX array,
	       global dimension (N, N),
	       local dimension ( LLD_Z, LOCc(JZ+N-1) )
	       If JOBZ = 'V', then on normal exit the first M columns of Z
	       contain the orthonormal eigenvectors of the matrix
	       corresponding to the selected eigenvalues.  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.

       IZ      (global input) INTEGER
	       Z's global row index, which points to the beginning of the
	       submatrix which is to be operated on.

       JZ      (global input) INTEGER
	       Z's global column index, which points to the beginning of
	       the submatrix which is to be operated on.

       DESCZ   (global and local input) INTEGER array of dimension DLEN_.
	       The array descriptor for the distributed matrix Z.
	       DESCZ( CTXT_ ) must equal DESCA( CTXT_ )

       WORK    (local workspace/output) COMPLEX array,
	       dimension (LWORK)
	       WORK(1) returns workspace adequate workspace to allow
	       optimal performance.

       LWORK   (local input) INTEGER
	       Size of WORK array.  If only eigenvalues are requested:
		 LWORK >= N + MAX( NB * ( NP0 + 1 ), 3 )
	       If eigenvectors are requested:
		 LWORK >= N + ( NP0 + MQ0 + NB ) * NB
	       with NQ0 = NUMROC( NN, NB, 0, 0, NPCOL ).

	       For optimal performance, greater workspace is needed, i.e.
		 LWORK >= MAX( LWORK, NHETRD_LWORK )
	       Where LWORK is as defined above, and
	       NHETRD_LWORK = N + 2*( ANB+1 )*( 4*NPS+2 ) +
		 ( NPS + 1 ) * NPS

	       ICTXT = DESCA( CTXT_ )
	       ANB = PJLAENV( ICTXT, 3, 'PCHETTRD', 'L', 0, 0, 0, 0 )
	       SQNPC = SQRT( DBLE( NPROW * NPCOL ) )
	       NPS = MAX( NUMROC( N, 1, 0, 0, SQNPC ), 2*ANB )

	       NUMROC is a ScaLAPACK tool functions;
	       PJLAENV is a ScaLAPACK envionmental inquiry function
	       MYROW, MYCOL, NPROW and NPCOL can be determined by calling
	       the subroutine BLACS_GRIDINFO.

	       If LWORK = -1, then LWORK is global input and a workspace
	       query is assumed; the routine only calculates the
	       optimal size for all work arrays. Each of these
	       values is returned in the first entry of the corresponding
	       work array, and no error message is issued by PXERBLA.

       RWORK   (local workspace/output) REAL array,
		  dimension (LRWORK)
	       On return, WROK(1) contains the optimal amount of
	       workspace required for efficient execution.
	       if JOBZ='N' RWORK(1) = optimal amount of workspace
		  required to compute eigenvalues efficiently
	       if JOBZ='V' RWORK(1) = optimal amount of workspace
		  required to compute eigenvalues and eigenvectors
		  efficiently with no guarantee on orthogonality.
		  If RANGE='V', it is assumed that all eigenvectors
		  may be required.

       LRWORK	(local input) INTEGER
	       Size of RWORK
	       See below for definitions of variables used to define LRWORK.
	       If no eigenvectors are requested (JOBZ = 'N') then
		  LRWORK >= 5 * NN + 4 * N
	       If eigenvectors are requested (JOBZ = 'V' ) then
		  the amount of workspace required to guarantee that all
		  eigenvectors are computed is:
		  LRWORK >= 4*N + MAX( 5*NN, NP0 * MQ0 ) +
		    ICEIL( NEIG, NPROW*NPCOL)*NN

		  The computed eigenvectors may not be orthogonal if the
		  minimal workspace is supplied and ORFAC is too small.
		  If you want to guarantee orthogonality (at the cost
		  of potentially poor performance) you should add
		  the following to LRWORK:
		     (CLUSTERSIZE-1)*N
		  where CLUSTERSIZE is the number of eigenvalues in the
		  largest cluster, where a cluster is defined as a set of
		  close eigenvalues: { W(K),...,W(K+CLUSTERSIZE-1) |
				       W(J+1) <= W(J) + ORFAC*2*norm(A) }
	       Variable definitions:
		  NEIG = number of eigenvectors requested
		  NB = DESCA( MB_ ) = DESCA( NB_ ) =
		       DESCZ( MB_ ) = DESCZ( NB_ )
		  NN = MAX( N, NB, 2 )
		  DESCA( RSRC_ ) = DESCA( NB_ ) = DESCZ( RSRC_ ) =
				   DESCZ( CSRC_ ) = 0
		  NP0 = NUMROC( NN, NB, 0, 0, NPROW )
		  MQ0 = NUMROC( MAX( NEIG, NB, 2 ), NB, 0, 0, NPCOL )
		  ICEIL( X, Y ) is a ScaLAPACK function returning
		  ceiling(X/Y)

	       When LRWORK is too small:
		  If LRWORK is too small to guarantee orthogonality,
		  PCHEEVX attempts to maintain orthogonality in
		  the clusters with the smallest
		  spacing between the eigenvalues.
		  If LRWORK is too small to compute all the eigenvectors
		  requested, no computation is performed and INFO=-25
		  is returned.	Note that when RANGE='V', PCHEEVX does
		  not know how many eigenvectors are requested until
		  the eigenvalues are computed.  Therefore, when RANGE='V'
		  and as long as LRWORK is large enough to allow PCHEEVX to
		  compute the eigenvalues, PCHEEVX will compute the
		  eigenvalues and as many eigenvectors as it can.

	       Relationship between workspace, orthogonality & performance:
		  If CLUSTERSIZE >= N/SQRT(NPROW*NPCOL), then providing
		  enough space to compute all the eigenvectors
		  orthogonally will cause serious degradation in
		  performance. In the limit (i.e. CLUSTERSIZE = N-1)
		  PCSTEIN will perform no better than CSTEIN on 1
		  processor.
		  For CLUSTERSIZE = N/SQRT(NPROW*NPCOL) reorthogonalizing
		  all eigenvectors will increase the total execution time
		  by a factor of 2 or more.
		  For CLUSTERSIZE > N/SQRT(NPROW*NPCOL) execution time will
		  grow as the square of the cluster size, all other factors
		  remaining equal and assuming enough workspace.  Less
		  workspace means less reorthogonalization but faster
		  execution.

	       If LRWORK = -1, then LRWORK is global input and a workspace
	       query is assumed; the routine only calculates the size
	       required for optimal performance for all work arrays. Each of
	       these values is returned in the first entry of the
	       corresponding work arrays, and no error message is issued by
	       PXERBLA.

       IWORK   (local workspace) INTEGER array
	       On return, IWORK(1) contains the amount of integer workspace
	       required.

       LIWORK  (local input) INTEGER
	       size of IWORK
	       LIWORK >= 6 * NNP
	       Where:
		 NNP = MAX( N, NPROW*NPCOL + 1, 4 )
	       If LIWORK = -1, then LIWORK is global input and a workspace
	       query is assumed; the routine only calculates the minimum
	       and optimal size for all work arrays. Each of these
	       values is returned in the first entry of the corresponding
	       work array, and no error message is issued by PXERBLA.

       IFAIL   (global output) INTEGER array, dimension (N)
	       If JOBZ = 'V', then on normal exit, the first M elements of
	       IFAIL are zero.	If (MOD(INFO,2).NE.0) on exit, then
	       IFAIL contains the
	       indices of the eigenvectors that failed to converge.
	       If JOBZ = 'N', then IFAIL is not referenced.

       ICLUSTR (global output) integer array, dimension (2*NPROW*NPCOL)
	       This array contains indices of eigenvectors corresponding to
	       a cluster of eigenvalues that could not be reorthogonalized
	       due to insufficient workspace (see LWORK, ORFAC and INFO).
	       Eigenvectors corresponding to clusters of eigenvalues indexed
	       ICLUSTR(2*I-1) to ICLUSTR(2*I), could not be
	       reorthogonalized due to lack of workspace. Hence the
	       eigenvectors corresponding to these clusters may not be
	       orthogonal.  ICLUSTR() is a zero terminated array.
	       (ICLUSTR(2*K).NE.0 .AND. ICLUSTR(2*K+1).EQ.0) if and only if
	       K is the number of clusters
	       ICLUSTR is not referenced if JOBZ = 'N'

       GAP     (global output) REAL array,
		  dimension (NPROW*NPCOL)
	       This array contains the gap between eigenvalues whose
	       eigenvectors could not be reorthogonalized. The output
	       values in this array correspond to the clusters indicated
	       by the array ICLUSTR. As a result, the dot product between
	       eigenvectors correspoding to the I^th cluster may be as high
	       as ( C * n ) / GAP(I) where C is a small constant.

       INFO    (global output) INTEGER
	       = 0:  successful exit
	       < 0:  If the i-th argument is an array and the j-entry had
		     an illegal value, then INFO = -(i*100+j), if the i-th
		     argument is a scalar and had an illegal value, then
		     INFO = -i.
	       > 0:  if (MOD(INFO,2).NE.0), then one or more eigenvectors
		       failed to converge.  Their indices are stored
		       in IFAIL.  Ensure ABSTOL=2.0*PSLAMCH( 'U' )
		       Send e-mail to scalapack@cs.utk.edu
		     if (MOD(INFO/2,2).NE.0),then eigenvectors corresponding
		       to one or more clusters of eigenvalues could not be
		       reorthogonalized because of insufficient workspace.
		       The indices of the clusters are stored in the array
		       ICLUSTR.
		     if (MOD(INFO/4,2).NE.0), then space limit prevented
		       PCHEEVX from computing all of the eigenvectors
		       between VL and VU.  The number of eigenvectors
		       computed is returned in NZ.
		     if (MOD(INFO/8,2).NE.0), then PCSTEBZ failed to compute
		       eigenvalues.  Ensure ABSTOL=2.0*PSLAMCH( 'U' )
		       Send e-mail to scalapack@cs.utk.edu

LAPACK version 1.5						    12 May 1997 							PCHEEVX(l)