sdisna.f

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      SUBROUTINE SDISNA( JOB, M, N, D, SEP, INFO )
*
*  -- LAPACK routine (version 3.2) --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*     November 2006
*
*     .. Scalar Arguments ..
      CHARACTER          JOB
      INTEGER            INFO, M, N
*     ..
*     .. Array Arguments ..
      REAL               D( * ), SEP( * )
*     ..
*
*  Purpose
*  =======
*
*  SDISNA computes the reciprocal condition numbers for the eigenvectors
*  of a real symmetric or complex Hermitian matrix or for the left or
*  right singular vectors of a general m-by-n matrix. The reciprocal
*  condition number is the 'gap' between the corresponding eigenvalue or
*  singular value and the nearest other one.
*
*  The bound on the error, measured by angle in radians, in the I-th
*  computed vector is given by
*
*         SLAMCH( 'E' ) * ( ANORM / SEP( I ) )
*
*  where ANORM = 2-norm(A) = max( abs( D(j) ) ).  SEP(I) is not allowed
*  to be smaller than SLAMCH( 'E' )*ANORM in order to limit the size of
*  the error bound.
*
*  SDISNA may also be used to compute error bounds for eigenvectors of
*  the generalized symmetric definite eigenproblem.
*
*  Arguments
*  =========
*
*  JOB     (input) CHARACTER*1
*          Specifies for which problem the reciprocal condition numbers
*          should be computed:
*          = 'E':  the eigenvectors of a symmetric/Hermitian matrix;
*          = 'L':  the left singular vectors of a general matrix;
*          = 'R':  the right singular vectors of a general matrix.
*
*  M       (input) INTEGER
*          The number of rows of the matrix. M >= 0.
*
*  N       (input) INTEGER
*          If JOB = 'L' or 'R', the number of columns of the matrix,
*          in which case N >= 0. Ignored if JOB = 'E'.
*
*  D       (input) REAL array, dimension (M) if JOB = 'E'
*                              dimension (min(M,N)) if JOB = 'L' or 'R'
*          The eigenvalues (if JOB = 'E') or singular values (if JOB =
*          'L' or 'R') of the matrix, in either increasing or decreasing
*          order. If singular values, they must be non-negative.
*
*  SEP     (output) REAL array, dimension (M) if JOB = 'E'
*                               dimension (min(M,N)) if JOB = 'L' or 'R'
*          The reciprocal condition numbers of the vectors.
*
*  INFO    (output) INTEGER
*          = 0:  successful exit.
*          < 0:  if INFO = -i, the i-th argument had an illegal value.
*
*  =====================================================================
*
*     .. Parameters ..
      REAL               ZERO
      PARAMETER          ( ZERO = 0.0E+0 )
*     ..
*     .. Local Scalars ..
      LOGICAL            DECR, EIGEN, INCR, LEFT, RIGHT, SING
      INTEGER            I, K
      REAL               ANORM, EPS, NEWGAP, OLDGAP, SAFMIN, THRESH
*     ..
*     .. External Functions ..
      LOGICAL            LSAME
      REAL               SLAMCH
      EXTERNAL           LSAME, SLAMCH
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          ABS, MAX, MIN
*     ..
*     .. External Subroutines ..
      EXTERNAL           XERBLA
*     ..
*     .. Executable Statements ..
*
*     Test the input arguments
*
      INFO = 0
      EIGEN = LSAME( JOB, 'E' )
      LEFT = LSAME( JOB, 'L' )
      RIGHT = LSAME( JOB, 'R' )
      SING = LEFT .OR. RIGHT
      IF( EIGEN ) THEN
         K = M
      ELSE IF( SING ) THEN
         K = MIN( M, N )
      END IF
      IF( .NOT.EIGEN .AND. .NOT.SING ) THEN
         INFO = -1
      ELSE IF( M.LT.0 ) THEN
         INFO = -2
      ELSE IF( K.LT.0 ) THEN
         INFO = -3
      ELSE
         INCR = .TRUE.
         DECR = .TRUE.
         DO 10 I = 1, K - 1
            IF( INCR )
     $         INCR = INCR .AND. D( I ).LE.D( I+1 )
            IF( DECR )
     $         DECR = DECR .AND. D( I ).GE.D( I+1 )
   10    CONTINUE
         IF( SING .AND. K.GT.0 ) THEN
            IF( INCR )
     $         INCR = INCR .AND. ZERO.LE.D( 1 )
            IF( DECR )
     $         DECR = DECR .AND. D( K ).GE.ZERO
         END IF
         IF( .NOT.( INCR .OR. DECR ) )
     $      INFO = -4
      END IF
      IF( INFO.NE.0 ) THEN
         CALL XERBLA( 'SDISNA', -INFO )
         RETURN
      END IF
*
*     Quick return if possible
*
      IF( K.EQ.0 )
     $   RETURN
*
*     Compute reciprocal condition numbers
*
      IF( K.EQ.1 ) THEN
         SEP( 1 ) = SLAMCH( 'O' )
      ELSE
         OLDGAP = ABS( D( 2 )-D( 1 ) )
         SEP( 1 ) = OLDGAP
         DO 20 I = 2, K - 1
            NEWGAP = ABS( D( I+1 )-D( I ) )
            SEP( I ) = MIN( OLDGAP, NEWGAP )
            OLDGAP = NEWGAP
   20    CONTINUE
         SEP( K ) = OLDGAP
      END IF
      IF( SING ) THEN
         IF( ( LEFT .AND. M.GT.N ) .OR. ( RIGHT .AND. M.LT.N ) ) THEN
            IF( INCR )
     $         SEP( 1 ) = MIN( SEP( 1 ), D( 1 ) )
            IF( DECR )
     $         SEP( K ) = MIN( SEP( K ), D( K ) )
         END IF
      END IF
*
*     Ensure that reciprocal condition numbers are not less than
*     threshold, in order to limit the size of the error bound
*
      EPS = SLAMCH( 'E' )
      SAFMIN = SLAMCH( 'S' )
      ANORM = MAX( ABS( D( 1 ) ), ABS( D( K ) ) )
      IF( ANORM.EQ.ZERO ) THEN
         THRESH = EPS
      ELSE
         THRESH = MAX( EPS*ANORM, SAFMIN )
      END IF
      DO 30 I = 1, K
         SEP( I ) = MAX( SEP( I ), THRESH )
   30 CONTINUE
*
      RETURN
*
*     End of SDISNA
*
      END