220 lines
7.2 KiB
Fortran
220 lines
7.2 KiB
Fortran
c-----------------------------------------------------------------------
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c\BeginDoc
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c
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c\Name: ssgets
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c
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c\Description:
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c Given the eigenvalues of the symmetric tridiagonal matrix H,
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c computes the NP shifts AMU that are zeros of the polynomial of
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c degree NP which filters out components of the unwanted eigenvectors
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c corresponding to the AMU's based on some given criteria.
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c
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c NOTE: This is called even in the case of user specified shifts in
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c order to sort the eigenvalues, and error bounds of H for later use.
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c
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c\Usage:
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c call ssgets
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c ( ISHIFT, WHICH, KEV, NP, RITZ, BOUNDS, SHIFTS )
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c
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c\Arguments
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c ISHIFT Integer. (INPUT)
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c Method for selecting the implicit shifts at each iteration.
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c ISHIFT = 0: user specified shifts
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c ISHIFT = 1: exact shift with respect to the matrix H.
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c
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c WHICH Character*2. (INPUT)
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c Shift selection criteria.
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c 'LM' -> KEV eigenvalues of largest magnitude are retained.
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c 'SM' -> KEV eigenvalues of smallest magnitude are retained.
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c 'LA' -> KEV eigenvalues of largest value are retained.
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c 'SA' -> KEV eigenvalues of smallest value are retained.
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c 'BE' -> KEV eigenvalues, half from each end of the spectrum.
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c If KEV is odd, compute one more from the high end.
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c
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c KEV Integer. (INPUT)
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c KEV+NP is the size of the matrix H.
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c
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c NP Integer. (INPUT)
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c Number of implicit shifts to be computed.
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c
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c RITZ Real array of length KEV+NP. (INPUT/OUTPUT)
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c On INPUT, RITZ contains the eigenvalues of H.
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c On OUTPUT, RITZ are sorted so that the unwanted eigenvalues
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c are in the first NP locations and the wanted part is in
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c the last KEV locations. When exact shifts are selected, the
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c unwanted part corresponds to the shifts to be applied.
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c
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c BOUNDS Real array of length KEV+NP. (INPUT/OUTPUT)
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c Error bounds corresponding to the ordering in RITZ.
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c
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c SHIFTS Real array of length NP. (INPUT/OUTPUT)
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c On INPUT: contains the user specified shifts if ISHIFT = 0.
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c On OUTPUT: contains the shifts sorted into decreasing order
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c of magnitude with respect to the Ritz estimates contained in
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c BOUNDS. If ISHIFT = 0, SHIFTS is not modified on exit.
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c
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c\EndDoc
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c
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c-----------------------------------------------------------------------
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c
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c\BeginLib
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c
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c\Local variables:
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c xxxxxx real
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c
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c\Routines called:
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c ssortr ARPACK utility sorting routine.
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c ivout ARPACK utility routine that prints integers.
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c second ARPACK utility routine for timing.
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c svout ARPACK utility routine that prints vectors.
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c scopy Level 1 BLAS that copies one vector to another.
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c sswap Level 1 BLAS that swaps the contents of two vectors.
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c
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c\Author
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c Danny Sorensen Phuong Vu
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c Richard Lehoucq CRPC / Rice University
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c Dept. of Computational & Houston, Texas
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c Applied Mathematics
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c Rice University
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c Houston, Texas
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c
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c\Revision history:
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c xx/xx/93: Version ' 2.1'
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c
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c\SCCS Information: @(#)
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c FILE: sgets.F SID: 2.4 DATE OF SID: 4/19/96 RELEASE: 2
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c
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c\Remarks
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c
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c\EndLib
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c
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c-----------------------------------------------------------------------
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c
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subroutine ssgets ( ishift, which, kev, np, ritz, bounds, shifts )
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c
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c %----------------------------------------------------%
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c | Include files for debugging and timing information |
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c %----------------------------------------------------%
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c
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include 'debug.h'
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include 'stat.h'
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c
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c %------------------%
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c | Scalar Arguments |
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c %------------------%
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c
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character*2 which
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integer ishift, kev, np
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c
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c %-----------------%
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c | Array Arguments |
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c %-----------------%
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c
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Real
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& bounds(kev+np), ritz(kev+np), shifts(np)
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c
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c %------------%
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c | Parameters |
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c %------------%
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c
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Real
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& one, zero
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parameter (one = 1.0E+0, zero = 0.0E+0)
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c
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c %---------------%
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c | Local Scalars |
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c %---------------%
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c
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integer kevd2, msglvl
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c
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c %----------------------%
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c | External Subroutines |
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c %----------------------%
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c
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external sswap, scopy, ssortr, second
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c
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c %---------------------%
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c | Intrinsic Functions |
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c %---------------------%
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c
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intrinsic max, min
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c
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c %-----------------------%
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c | Executable Statements |
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c %-----------------------%
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c
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c %-------------------------------%
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c | Initialize timing statistics |
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c | & message level for debugging |
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c %-------------------------------%
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c
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call second (t0)
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msglvl = msgets
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c
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if (which .eq. 'BE') then
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c
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c %-----------------------------------------------------%
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c | Both ends of the spectrum are requested. |
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c | Sort the eigenvalues into algebraically increasing |
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c | order first then swap high end of the spectrum next |
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c | to low end in appropriate locations. |
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c | NOTE: when np < floor(kev/2) be careful not to swap |
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c | overlapping locations. |
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c %-----------------------------------------------------%
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c
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call ssortr ('LA', .true., kev+np, ritz, bounds)
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kevd2 = kev / 2
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if ( kev .gt. 1 ) then
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call sswap ( min(kevd2,np), ritz, 1,
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& ritz( max(kevd2,np)+1 ), 1)
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call sswap ( min(kevd2,np), bounds, 1,
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& bounds( max(kevd2,np)+1 ), 1)
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end if
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c
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else
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c
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c %----------------------------------------------------%
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c | LM, SM, LA, SA case. |
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c | Sort the eigenvalues of H into the desired order |
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c | and apply the resulting order to BOUNDS. |
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c | The eigenvalues are sorted so that the wanted part |
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c | are always in the last KEV locations. |
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c %----------------------------------------------------%
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c
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call ssortr (which, .true., kev+np, ritz, bounds)
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end if
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c
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if (ishift .eq. 1 .and. np .gt. 0) then
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c
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c %-------------------------------------------------------%
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c | Sort the unwanted Ritz values used as shifts so that |
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c | the ones with largest Ritz estimates are first. |
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c | This will tend to minimize the effects of the |
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c | forward instability of the iteration when the shifts |
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c | are applied in subroutine ssapps. |
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c %-------------------------------------------------------%
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c
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call ssortr ('SM', .true., np, bounds, ritz)
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call scopy (np, ritz, 1, shifts, 1)
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end if
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c
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call second (t1)
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tsgets = tsgets + (t1 - t0)
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c
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if (msglvl .gt. 0) then
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call ivout (logfil, 1, kev, ndigit, '_sgets: KEV is')
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call ivout (logfil, 1, np, ndigit, '_sgets: NP is')
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call svout (logfil, kev+np, ritz, ndigit,
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& '_sgets: Eigenvalues of current H matrix')
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call svout (logfil, kev+np, bounds, ndigit,
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& '_sgets: Associated Ritz estimates')
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end if
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c
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return
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c
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c %---------------%
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c | End of ssgets |
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c %---------------%
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c
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end
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