sget53()

subroutine sget53	(	real, dimension( lda, * )	a,
		integer	lda,
		real, dimension( ldb, * )	b,
		integer	ldb,
		real	scale,
		real	wr,
		real	wi,
		real	result,
		integer	info )

SGET53

Purpose:

!>
!> SGET53  checks the generalized eigenvalues computed by SLAG2.
!>
!> The basic test for an eigenvalue is:
!>
!>                              | det( s A - w B ) |
!>     RESULT =  ---------------------------------------------------
!>               ulp max( s norm(A), |w| norm(B) )*norm( s A - w B )
!>
!> Two  are performed:
!>
!> (1)  ulp*max( s*norm(A), |w|*norm(B) )  must be at least
!>      safe_minimum.  This insures that the test performed is
!>      not essentially  det(0*A + 0*B)=0.
!>
!> (2)  s*norm(A) + |w|*norm(B) must be less than 1/safe_minimum.
!>      This insures that  s*A - w*B  will not overflow.
!>
!> If these tests are not passed, then  s  and  w  are scaled and
!> tested anyway, if this is possible.
!> 

Parameters

[in]	A	!> A is REAL array, dimension (LDA, 2) !> The 2x2 matrix A. !>
[in]	LDA	!> LDA is INTEGER !> The leading dimension of A. It must be at least 2. !>
[in]	B	!> B is REAL array, dimension (LDB, N) !> The 2x2 upper-triangular matrix B. !>
[in]	LDB	!> LDB is INTEGER !> The leading dimension of B. It must be at least 2. !>
[in]	SCALE	!> SCALE is REAL !> The s in the formula s A - w B . It is !> assumed to be non-negative. !>
[in]	WR	!> WR is REAL !> The real part of the eigenvalue w in the formula !> s A - w B . !>
[in]	WI	!> WI is REAL !> The imaginary part of the eigenvalue w in the formula !> s A - w B . !>
[out]	RESULT	!> RESULT is REAL !> If INFO is 2 or less, the value computed by the test !> described above. !> If INFO=3, this will just be 1/ulp. !>
[out]	INFO	!> INFO is INTEGER !> =0: The input data pass the . !> =1: s*norm(A) + |w|*norm(B) > 1/safe_minimum. !> =2: ulp*max( s*norm(A), |w|*norm(B) ) < safe_minimum !> =3: same as INFO=2, but s and w could not be scaled so !> as to compute the test. !>

Author

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Definition at line 125 of file sget53.f.

*
*  -- LAPACK test 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, LDA, LDB
      REAL               RESULT, SCALE, WI, WR
*     ..
*     .. Array Arguments ..
      REAL               A( LDA, * ), B( LDB, * )
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      REAL               ZERO, ONE
      parameter( zero = 0.0, one = 1.0 )
*     ..
*     .. Local Scalars ..
      REAL               ABSW, ANORM, BNORM, CI11, CI12, CI22, CNORM,
     $                   CR11, CR12, CR21, CR22, CSCALE, DETI, DETR, S1,
     $                   SAFMIN, SCALES, SIGMIN, TEMP, ULP, WIS, WRS
*     ..
*     .. External Functions ..
      REAL               SLAMCH
      EXTERNAL           slamch
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          abs, max, sqrt
*     ..
*     .. Executable Statements ..
*
*     Initialize
*
      info = 0
      result = zero
      scales = scale
      wrs = wr
      wis = wi
*
*     Machine constants and norms
*
      safmin = slamch( 'Safe minimum' )
      ulp = slamch( 'Epsilon' )*slamch( 'Base' )
      absw = abs( wrs ) + abs( wis )
      anorm = max( abs( a( 1, 1 ) )+abs( a( 2, 1 ) ),
     $        abs( a( 1, 2 ) )+abs( a( 2, 2 ) ), safmin )
      bnorm = max( abs( b( 1, 1 ) ), abs( b( 1, 2 ) )+abs( b( 2, 2 ) ),
     $        safmin )
*
*     Check for possible overflow.
*
      temp = ( safmin*bnorm )*absw + ( safmin*anorm )*scales
      IF( temp.GE.one ) THEN
*
*        Scale down to avoid overflow
*
         info = 1
         temp = one / temp
         scales = scales*temp
         wrs = wrs*temp
         wis = wis*temp
         absw = abs( wrs ) + abs( wis )
      END IF
      s1 = max( ulp*max( scales*anorm, absw*bnorm ),
     $     safmin*max( scales, absw ) )
*
*     Check for W and SCALE essentially zero.
*
      IF( s1.LT.safmin ) THEN
         info = 2
         IF( scales.LT.safmin .AND. absw.LT.safmin ) THEN
            info = 3
            result = one / ulp
            RETURN
         END IF
*
*        Scale up to avoid underflow
*
         temp = one / max( scales*anorm+absw*bnorm, safmin )
         scales = scales*temp
         wrs = wrs*temp
         wis = wis*temp
         absw = abs( wrs ) + abs( wis )
         s1 = max( ulp*max( scales*anorm, absw*bnorm ),
     $        safmin*max( scales, absw ) )
         IF( s1.LT.safmin ) THEN
            info = 3
            result = one / ulp
            RETURN
         END IF
      END IF
*
*     Compute C = s A - w B
*
      cr11 = scales*a( 1, 1 ) - wrs*b( 1, 1 )
      ci11 = -wis*b( 1, 1 )
      cr21 = scales*a( 2, 1 )
      cr12 = scales*a( 1, 2 ) - wrs*b( 1, 2 )
      ci12 = -wis*b( 1, 2 )
      cr22 = scales*a( 2, 2 ) - wrs*b( 2, 2 )
      ci22 = -wis*b( 2, 2 )
*
*     Compute the smallest singular value of s A - w B:
*
*                 |det( s A - w B )|
*     sigma_min = ------------------
*                 norm( s A - w B )
*
      cnorm = max( abs( cr11 )+abs( ci11 )+abs( cr21 ),
     $        abs( cr12 )+abs( ci12 )+abs( cr22 )+abs( ci22 ), safmin )
      cscale = one / sqrt( cnorm )
      detr = ( cscale*cr11 )*( cscale*cr22 ) -
     $       ( cscale*ci11 )*( cscale*ci22 ) -
     $       ( cscale*cr12 )*( cscale*cr21 )
      deti = ( cscale*cr11 )*( cscale*ci22 ) +
     $       ( cscale*ci11 )*( cscale*cr22 ) -
     $       ( cscale*ci12 )*( cscale*cr21 )
      sigmin = abs( detr ) + abs( deti )
      result = sigmin / s1
      RETURN
*
*     End of SGET53
*

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