slarra.f

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*> \brief \b SLARRA computes the splitting points with the specified threshold.
*
*  =========== DOCUMENTATION ===========
*
* Online html documentation available at
*            http://www.netlib.org/lapack/explore-html/
*
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*> \endhtmlonly
*
*  Definition:
*  ===========
*
*       SUBROUTINE SLARRA( N, D, E, E2, SPLTOL, TNRM,
*                           NSPLIT, ISPLIT, INFO )
*
*       .. Scalar Arguments ..
*       INTEGER            INFO, N, NSPLIT
*       REAL                SPLTOL, TNRM
*       ..
*       .. Array Arguments ..
*       INTEGER            ISPLIT( * )
*       REAL               D( * ), E( * ), E2( * )
*       ..
*
*
*> \par Purpose:
*  =============
*>
*> \verbatim
*>
*> Compute the splitting points with threshold SPLTOL.
*> SLARRA sets any "small" off-diagonal elements to zero.
*> \endverbatim
*
*  Arguments:
*  ==========
*
*> \param[in] N
*> \verbatim
*>          N is INTEGER
*>          The order of the matrix. N > 0.
*> \endverbatim
*>
*> \param[in] D
*> \verbatim
*>          D is REAL array, dimension (N)
*>          On entry, the N diagonal elements of the tridiagonal
*>          matrix T.
*> \endverbatim
*>
*> \param[in,out] E
*> \verbatim
*>          E is REAL array, dimension (N)
*>          On entry, the first (N-1) entries contain the subdiagonal
*>          elements of the tridiagonal matrix T; E(N) need not be set.
*>          On exit, the entries E( ISPLIT( I ) ), 1 <= I <= NSPLIT,
*>          are set to zero, the other entries of E are untouched.
*> \endverbatim
*>
*> \param[in,out] E2
*> \verbatim
*>          E2 is REAL array, dimension (N)
*>          On entry, the first (N-1) entries contain the SQUARES of the
*>          subdiagonal elements of the tridiagonal matrix T;
*>          E2(N) need not be set.
*>          On exit, the entries E2( ISPLIT( I ) ),
*>          1 <= I <= NSPLIT, have been set to zero
*> \endverbatim
*>
*> \param[in] SPLTOL
*> \verbatim
*>          SPLTOL is REAL
*>          The threshold for splitting. Two criteria can be used:
*>          SPLTOL<0 : criterion based on absolute off-diagonal value
*>          SPLTOL>0 : criterion that preserves relative accuracy
*> \endverbatim
*>
*> \param[in] TNRM
*> \verbatim
*>          TNRM is REAL
*>          The norm of the matrix.
*> \endverbatim
*>
*> \param[out] NSPLIT
*> \verbatim
*>          NSPLIT is INTEGER
*>          The number of blocks T splits into. 1 <= NSPLIT <= N.
*> \endverbatim
*>
*> \param[out] ISPLIT
*> \verbatim
*>          ISPLIT is INTEGER array, dimension (N)
*>          The splitting points, at which T breaks up into blocks.
*>          The first block consists of rows/columns 1 to ISPLIT(1),
*>          the second of rows/columns ISPLIT(1)+1 through ISPLIT(2),
*>          etc., and the NSPLIT-th consists of rows/columns
*>          ISPLIT(NSPLIT-1)+1 through ISPLIT(NSPLIT)=N.
*> \endverbatim
*>
*> \param[out] INFO
*> \verbatim
*>          INFO is INTEGER
*>          = 0:  successful exit
*> \endverbatim
*
*  Authors:
*  ========
*
*> \author Univ. of Tennessee
*> \author Univ. of California Berkeley
*> \author Univ. of Colorado Denver
*> \author NAG Ltd.
*
*> \ingroup larra
*
*> \par Contributors:
*  ==================
*>
*> Beresford Parlett, University of California, Berkeley, USA \n
*> Jim Demmel, University of California, Berkeley, USA \n
*> Inderjit Dhillon, University of Texas, Austin, USA \n
*> Osni Marques, LBNL/NERSC, USA \n
*> Christof Voemel, University of California, Berkeley, USA
*
*  =====================================================================
      SUBROUTINE slarra( N, D, E, E2, SPLTOL, TNRM,
     $                    NSPLIT, ISPLIT, INFO )
*
*  -- LAPACK auxiliary routine --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*
*     .. Scalar Arguments ..
      INTEGER            INFO, N, NSPLIT
      REAL                SPLTOL, TNRM
*     ..
*     .. Array Arguments ..
      INTEGER            ISPLIT( * )
      REAL               D( * ), E( * ), E2( * )
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      REAL               ZERO
      parameter( zero = 0.0e0 )
*     ..
*     .. Local Scalars ..
      INTEGER            I
      REAL               EABS, TMP1
 
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          abs
*     ..
*     .. Executable Statements ..
*
      info = 0
      nsplit = 1
*
*     Quick return if possible
*
      IF( n.LE.0 ) THEN
         RETURN
      END IF
*
*     Compute splitting points
      IF(spltol.LT.zero) THEN
*        Criterion based on absolute off-diagonal value
         tmp1 = abs(spltol)* tnrm
         DO 9 i = 1, n-1
            eabs = abs( e(i) )
            IF( eabs .LE. tmp1) THEN
               e(i) = zero
               e2(i) = zero
               isplit( nsplit ) = i
               nsplit = nsplit + 1
            END IF
 9       CONTINUE
      ELSE
*        Criterion that guarantees relative accuracy
         DO 10 i = 1, n-1
            eabs = abs( e(i) )
            IF( eabs .LE. spltol * sqrt(abs(d(i)))*sqrt(abs(d(i+1))) )
     $      THEN
               e(i) = zero
               e2(i) = zero
               isplit( nsplit ) = i
               nsplit = nsplit + 1
            END IF
 10      CONTINUE
      ENDIF
      isplit( nsplit ) = n
 
      RETURN
*
*     End of SLARRA
*
      END