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SUBROUTINE SSVDCT( N, S, E, SHIFT, NUM )
*
* -- LAPACK test routine (version 3.0) --
* Univ. of Tennessee, Univ. of California Berkeley, NAG Ltd.,
* Courant Institute, Argonne National Lab, and Rice University
* June 30, 1999
*
* .. Scalar Arguments ..
INTEGER N, NUM
REAL SHIFT
* ..
* .. Array Arguments ..
REAL E( * ), S( * )
* ..
*
* Purpose
* =======
*
* SSVDCT counts the number NUM of eigenvalues of a 2*N by 2*N
* tridiagonal matrix T which are less than or equal to SHIFT. T is
* formed by putting zeros on the diagonal and making the off-diagonals
* equal to S(1), E(1), S(2), E(2), ... , E(N-1), S(N). If SHIFT is
* positive, NUM is equal to N plus the number of singular values of a
* bidiagonal matrix B less than or equal to SHIFT. Here B has diagonal
* entries S(1), ..., S(N) and superdiagonal entries E(1), ... E(N-1).
* If SHIFT is negative, NUM is equal to the number of singular values
* of B greater than or equal to -SHIFT.
*
* See W. Kahan "Accurate Eigenvalues of a Symmetric Tridiagonal
* Matrix", Report CS41, Computer Science Dept., Stanford University,
* July 21, 1966
*
* Arguments
* =========
*
* N (input) INTEGER
* The dimension of the bidiagonal matrix B.
*
* S (input) REAL array, dimension (N)
* The diagonal entries of the bidiagonal matrix B.
*
* E (input) REAL array of dimension (N-1)
* The superdiagonal entries of the bidiagonal matrix B.
*
* SHIFT (input) REAL
* The shift, used as described under Purpose.
*
* NUM (output) INTEGER
* The number of eigenvalues of T less than or equal to SHIFT.
*
* =====================================================================
*
* .. Parameters ..
REAL ONE
PARAMETER ( ONE = 1.0E0 )
REAL ZERO
PARAMETER ( ZERO = 0.0E0 )
* ..
* .. Local Scalars ..
INTEGER I
REAL M1, M2, MX, OVFL, SOV, SSHIFT, SSUN, SUN, TMP,
$ TOM, U, UNFL
* ..
* .. External Functions ..
REAL SLAMCH
EXTERNAL SLAMCH
* ..
* .. Intrinsic Functions ..
INTRINSIC ABS, MAX, SQRT
* ..
* .. Executable Statements ..
*
* Get machine constants
*
UNFL = 2*SLAMCH( 'Safe minimum' )
OVFL = ONE / UNFL
*
* Find largest entry
*
MX = ABS( S( 1 ) )
DO 10 I = 1, N - 1
MX = MAX( MX, ABS( S( I+1 ) ), ABS( E( I ) ) )
10 CONTINUE
*
IF( MX.EQ.ZERO ) THEN
IF( SHIFT.LT.ZERO ) THEN
NUM = 0
ELSE
NUM = 2*N
END IF
RETURN
END IF
*
* Compute scale factors as in Kahan's report
*
SUN = SQRT( UNFL )
SSUN = SQRT( SUN )
SOV = SQRT( OVFL )
TOM = SSUN*SOV
IF( MX.LE.ONE ) THEN
M1 = ONE / MX
M2 = TOM
ELSE
M1 = ONE
M2 = TOM / MX
END IF
*
* Begin counting
*
U = ONE
NUM = 0
SSHIFT = ( SHIFT*M1 )*M2
U = -SSHIFT
IF( U.LE.SUN ) THEN
IF( U.LE.ZERO ) THEN
NUM = NUM + 1
IF( U.GT.-SUN )
$ U = -SUN
ELSE
U = SUN
END IF
END IF
TMP = ( S( 1 )*M1 )*M2
U = -TMP*( TMP / U ) - SSHIFT
IF( U.LE.SUN ) THEN
IF( U.LE.ZERO ) THEN
NUM = NUM + 1
IF( U.GT.-SUN )
$ U = -SUN
ELSE
U = SUN
END IF
END IF
DO 20 I = 1, N - 1
TMP = ( E( I )*M1 )*M2
U = -TMP*( TMP / U ) - SSHIFT
IF( U.LE.SUN ) THEN
IF( U.LE.ZERO ) THEN
NUM = NUM + 1
IF( U.GT.-SUN )
$ U = -SUN
ELSE
U = SUN
END IF
END IF
TMP = ( S( I+1 )*M1 )*M2
U = -TMP*( TMP / U ) - SSHIFT
IF( U.LE.SUN ) THEN
IF( U.LE.ZERO ) THEN
NUM = NUM + 1
IF( U.GT.-SUN )
$ U = -SUN
ELSE
U = SUN
END IF
END IF
20 CONTINUE
RETURN
*
* End of SSVDCT
*
END
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