135 lines
3.4 KiB
FortranFixed
135 lines
3.4 KiB
FortranFixed
SUBROUTINE DSTECT( N, A, B, SHIFT, NUM )
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*
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* -- LAPACK test routine (version 3.1) --
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* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
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* November 2006
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*
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* .. Scalar Arguments ..
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INTEGER N, NUM
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DOUBLE PRECISION SHIFT
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* ..
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* .. Array Arguments ..
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DOUBLE PRECISION A( * ), B( * )
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* ..
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*
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* Purpose
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* =======
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*
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* DSTECT counts the number NUM of eigenvalues of a tridiagonal
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* matrix T which are less than or equal to SHIFT. T has
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* diagonal entries A(1), ... , A(N), and offdiagonal entries
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* B(1), ..., B(N-1).
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* See W. Kahan "Accurate Eigenvalues of a Symmetric Tridiagonal
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* Matrix", Report CS41, Computer Science Dept., Stanford
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* University, July 21, 1966
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*
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* Arguments
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* =========
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*
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* N (input) INTEGER
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* The dimension of the tridiagonal matrix T.
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*
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* A (input) DOUBLE PRECISION array, dimension (N)
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* The diagonal entries of the tridiagonal matrix T.
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*
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* B (input) DOUBLE PRECISION array, dimension (N-1)
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* The offdiagonal entries of the tridiagonal matrix T.
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*
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* SHIFT (input) DOUBLE PRECISION
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* The shift, used as described under Purpose.
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*
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* NUM (output) INTEGER
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* The number of eigenvalues of T less than or equal
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* to SHIFT.
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*
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* =====================================================================
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*
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* .. Parameters ..
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DOUBLE PRECISION ZERO, ONE, THREE
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PARAMETER ( ZERO = 0.0D0, ONE = 1.0D0, THREE = 3.0D0 )
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* ..
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* .. Local Scalars ..
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INTEGER I
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DOUBLE PRECISION M1, M2, MX, OVFL, SOV, SSHIFT, SSUN, SUN, TMP,
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$ TOM, U, UNFL
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* ..
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* .. External Functions ..
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DOUBLE PRECISION DLAMCH
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EXTERNAL DLAMCH
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* ..
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* .. Intrinsic Functions ..
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INTRINSIC ABS, MAX, SQRT
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* ..
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* .. Executable Statements ..
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*
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* Get machine constants
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*
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UNFL = DLAMCH( 'Safe minimum' )
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OVFL = DLAMCH( 'Overflow' )
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*
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* Find largest entry
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*
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MX = ABS( A( 1 ) )
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DO 10 I = 1, N - 1
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MX = MAX( MX, ABS( A( I+1 ) ), ABS( B( I ) ) )
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10 CONTINUE
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*
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* Handle easy cases, including zero matrix
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*
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IF( SHIFT.GE.THREE*MX ) THEN
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NUM = N
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RETURN
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END IF
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IF( SHIFT.LT.-THREE*MX ) THEN
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NUM = 0
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RETURN
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END IF
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*
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* Compute scale factors as in Kahan's report
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* At this point, MX .NE. 0 so we can divide by it
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*
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SUN = SQRT( UNFL )
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SSUN = SQRT( SUN )
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SOV = SQRT( OVFL )
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TOM = SSUN*SOV
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IF( MX.LE.ONE ) THEN
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M1 = ONE / MX
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M2 = TOM
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ELSE
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M1 = ONE
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M2 = TOM / MX
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END IF
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*
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* Begin counting
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*
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NUM = 0
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SSHIFT = ( SHIFT*M1 )*M2
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U = ( A( 1 )*M1 )*M2 - SSHIFT
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IF( U.LE.SUN ) THEN
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IF( U.LE.ZERO ) THEN
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NUM = NUM + 1
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IF( U.GT.-SUN )
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$ U = -SUN
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ELSE
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U = SUN
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END IF
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END IF
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DO 20 I = 2, N
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TMP = ( B( I-1 )*M1 )*M2
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U = ( ( A( I )*M1 )*M2-TMP*( TMP / U ) ) - SSHIFT
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IF( U.LE.SUN ) THEN
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IF( U.LE.ZERO ) THEN
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NUM = NUM + 1
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IF( U.GT.-SUN )
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$ U = -SUN
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ELSE
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U = SUN
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END IF
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END IF
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20 CONTINUE
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RETURN
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*
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* End of DSTECT
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*
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END
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