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c\Name: znaupd
c  Reverse communication interface for the Implicitly Restarted Arnoldi
c  iteration. This is intended to be used to find a few eigenpairs of a 
c  complex linear operator OP with respect to a semi-inner product defined 
c  by a hermitian positive semi-definite real matrix B. B may be the identity 
c  matrix.  NOTE: if both OP and B are real, then dsaupd or dnaupd should
c  be used.
c  The computed approximate eigenvalues are called Ritz values and
c  the corresponding approximate eigenvectors are called Ritz vectors.
c  znaupd is usually called iteratively to solve one of the 
c  following problems:
c  Mode 1:  A*x = lambda*x.
c           ===> OP = A  and  B = I.
c  Mode 2:  A*x = lambda*M*x, M symmetric positive definite
c           ===> OP = inv[M]*A  and  B = M.
c           ===> (If M can be factored see remark 3 below)
c  Mode 3:  A*x = lambda*M*x, M symmetric semi-definite
c           ===> OP =  inv[A - sigma*M]*M   and  B = M. 
c           ===> shift-and-invert mode 
c           If OP*x = amu*x, then lambda = sigma + 1/amu.
c  NOTE: The action of w <- inv[A - sigma*M]*v or w <- inv[M]*v
c        should be accomplished either by a direct method
c        using a sparse matrix factorization and solving
c           [A - sigma*M]*w = v  or M*w = v,
c        or through an iterative method for solving these
c        systems.  If an iterative method is used, the
c        convergence test must be more stringent than
c        the accuracy requirements for the eigenvalue
c        approximations.
c  call znaupd
c  IDO     Integer.  (INPUT/OUTPUT)
c          Reverse communication flag.  IDO must be zero on the first 
c          call to znaupd.  IDO will be set internally to
c          indicate the type of operation to be performed.  Control is
c          then given back to the calling routine which has the
c          responsibility to carry out the requested operation and call
c          znaupd with the result.  The operand is given in
c          WORKD(IPNTR(1)), the result must be put in WORKD(IPNTR(2)).
c          -------------------------------------------------------------
c          IDO =  0: first call to the reverse communication interface
c          IDO = -1: compute  Y = OP * X  where
c                    IPNTR(1) is the pointer into WORKD for X,
c                    IPNTR(2) is the pointer into WORKD for Y.
c                    This is for the initialization phase to force the
c                    starting vector into the range of OP.
c          IDO =  1: compute  Y = OP * Z  and Z = B * X where
c                    IPNTR(1) is the pointer into WORKD for X,
c                    IPNTR(2) is the pointer into WORKD for Y,
c                    IPNTR(3) is the pointer into WORKD for Z.
c          IDO =  2: compute  Y = M * X  where
c                    IPNTR(1) is the pointer into WORKD for X,
c                    IPNTR(2) is the pointer into WORKD for Y.
c          IDO =  3: compute and return the shifts in the first 
c                    NP locations of WORKL.
c          IDO =  4: compute Z = OP * X
c          IDO = 99: done
c          -------------------------------------------------------------
c          After the initialization phase, when the routine is used in 
c          the "shift-and-invert" mode, the vector M * X is already 
c          available and does not need to be recomputed in forming OP*X.
c  BMAT    Character*1.  (INPUT)
c          BMAT specifies the type of the matrix B that defines the
c          semi-inner product for the operator OP.
c          BMAT = 'I' -> standard eigenvalue problem A*x = lambda*x
c          BMAT = 'G' -> generalized eigenvalue problem A*x = lambda*M*x
c  N       Integer.  (INPUT)
c          Dimension of the eigenproblem.
c  WHICH   Character*2.  (INPUT)
c          'LM' -> want the NEV eigenvalues of largest magnitude.
c          'SM' -> want the NEV eigenvalues of smallest magnitude.
c          'LR' -> want the NEV eigenvalues of largest real part.
c          'SR' -> want the NEV eigenvalues of smallest real part.
c          'LI' -> want the NEV eigenvalues of largest imaginary part.
c          'SI' -> want the NEV eigenvalues of smallest imaginary part.
c  NEV     Integer.  (INPUT)
c          Number of eigenvalues of OP to be computed. 0 < NEV < N-1.
c  TOL     Double precision  scalar.  (INPUT)
c          Stopping criteria: the relative accuracy of the Ritz value 
c          is considered acceptable if BOUNDS(I) .LE. TOL*ABS(RITZ(I))
c          where ABS(RITZ(I)) is the magnitude when RITZ(I) is complex.
c          DEFAULT = dlamch('EPS')  (machine precision as computed
c                    by the LAPACK auxiliary subroutine dlamch).
c  RESID   Complex*16 array of length N.  (INPUT/OUTPUT)
c          On INPUT: 
c          If INFO .EQ. 0, a random initial residual vector is used.
c          If INFO .NE. 0, RESID contains the initial residual vector,
c                          possibly from a previous run.
c          On OUTPUT:
c          RESID contains the final residual vector.
c  NCV     Integer.  (INPUT)
c          Number of columns of the matrix V. NCV must satisfy the two
c          inequalities 2 <= NCV-NEV and NCV <= N.
c          This will indicate how many Arnoldi vectors are generated 
c          at each iteration.  After the startup phase in which NEV 
c          Arnoldi vectors are generated, the algorithm generates 
c          approximately NCV-NEV Arnoldi vectors at each subsequent update 
c          iteration. Most of the cost in generating each Arnoldi vector is 
c          in the matrix-vector operation OP*x. 
c          NOTE: 2 <= NCV-NEV in order that complex conjugate pairs of Ritz 
c          values are kept together. (See remark 4 below)
c  V       Complex*16 array N by NCV.  (OUTPUT)
c          Contains the final set of Arnoldi basis vectors. 
c  LDV     Integer.  (INPUT)
c          Leading dimension of V exactly as declared in the calling program.
c  IPARAM  Integer array of length 11.  (INPUT/OUTPUT)
c          IPARAM(1) = ISHIFT: method for selecting the implicit shifts.
c          The shifts selected at each iteration are used to filter out
c          the components of the unwanted eigenvector.
c          -------------------------------------------------------------
c          ISHIFT = 0: the shifts are to be provided by the user via
c                      reverse communication.  The NCV eigenvalues of 
c                      the Hessenberg matrix H are returned in the part
c                      of WORKL array corresponding to RITZ.
c          ISHIFT = 1: exact shifts with respect to the current
c                      Hessenberg matrix H.  This is equivalent to 
c                      restarting the iteration from the beginning 
c                      after updating the starting vector with a linear
c                      combination of Ritz vectors associated with the 
c                      "wanted" eigenvalues.
c          ISHIFT = 2: other choice of internal shift to be defined.
c          -------------------------------------------------------------
c          IPARAM(2) = No longer referenced 
c          IPARAM(3) = MXITER
c          On INPUT:  maximum number of Arnoldi update iterations allowed. 
c          On OUTPUT: actual number of Arnoldi update iterations taken. 
c          IPARAM(4) = NB: blocksize to be used in the recurrence.
c          The code currently works only for NB = 1.
c          IPARAM(5) = NCONV: number of "converged" Ritz values.
c          This represents the number of Ritz values that satisfy
c          the convergence criterion.
c          IPARAM(6) = IUPD
c          No longer referenced. Implicit restarting is ALWAYS used.  
c          IPARAM(7) = MODE
c          On INPUT determines what type of eigenproblem is being solved.
c          Must be 1,2,3,4; See under \Description of znaupd for the 
c          four modes available.
c          IPARAM(8) = NP
c          When ido = 3 and the user provides shifts through reverse
c          communication (IPARAM(1)=0), _naupd returns NP, the number
c          of shifts the user is to provide. 0 < NP < NCV-NEV.
c          IPARAM(9) = NUMOP, IPARAM(10) = NUMOPB, IPARAM(11) = NUMREO,
c          OUTPUT: NUMOP  = total number of OP*x operations,
c                  NUMOPB = total number of B*x operations if BMAT='G',
c                  NUMREO = total number of steps of re-orthogonalization.
c  IPNTR   Integer array of length 14.  (OUTPUT)
c          Pointer to mark the starting locations in the WORKD and WORKL
c          arrays for matrices/vectors used by the Arnoldi iteration.
c          -------------------------------------------------------------
c          IPNTR(1): pointer to the current operand vector X in WORKD.
c          IPNTR(2): pointer to the current result vector Y in WORKD.
c          IPNTR(3): pointer to the vector B * X in WORKD when used in 
c                    the shift-and-invert mode.
c          IPNTR(4): pointer to the next available location in WORKL
c                    that is untouched by the program.
c          IPNTR(5): pointer to the NCV by NCV upper Hessenberg
c                    matrix H in WORKL.
c          IPNTR(6): pointer to the  ritz value array  RITZ
c          IPNTR(7): pointer to the (projected) ritz vector array Q
c          IPNTR(8): pointer to the error BOUNDS array in WORKL.
c          Note: IPNTR(9:13) is only referenced by zneupd. See Remark 2 below.
c          IPNTR(9): pointer to the NCV RITZ values of the 
c                    original system.
c          IPNTR(10): Not Used
c          IPNTR(11): pointer to the NCV corresponding error bounds.
c          IPNTR(14): pointer to the NP shifts in WORKL. See Remark 5 below.
c          -------------------------------------------------------------
c  WORKD   Complex*16 work array of length 3*N.  (REVERSE COMMUNICATION)
c          Distributed array to be used in the basic Arnoldi iteration
c          for reverse communication.  The user should not use WORKD 
c          as temporary workspace during the iteration !!!!!!!!!!
c          See Data Distribution Note below.  
c  WORKL   Complex*16 work array of length LWORKL.  (OUTPUT/WORKSPACE)
c          Private (replicated) array on each PE or array allocated on
c          the front end.  See Data Distribution Note below.
c  LWORKL  Integer.  (INPUT)
c          LWORKL must be at least 3*NCV**2 + 5*NCV.
c  RWORK   Double precision  work array of length NCV (WORKSPACE)
c          Private (replicated) array on each PE or array allocated on
c          the front end.
c  INFO    Integer.  (INPUT/OUTPUT)
c          If INFO .EQ. 0, a randomly initial residual vector is used.
c          If INFO .NE. 0, RESID contains the initial residual vector,
c                          possibly from a previous run.
c          Error flag on output.
c          =  0: Normal exit.
c          =  1: Maximum number of iterations taken.
c                All possible eigenvalues of OP has been found. IPARAM(5)  
c                returns the number of wanted converged Ritz values.
c          =  2: No longer an informational error. Deprecated starting
c                with release 2 of ARPACK.
c          =  3: No shifts could be applied during a cycle of the 
c                Implicitly restarted Arnoldi iteration. One possibility 
c                is to increase the size of NCV relative to NEV. 
c                See remark 4 below.
c          = -1: N must be positive.
c          = -2: NEV must be positive.
c          = -3: NCV-NEV >= 2 and less than or equal to N.
c          = -4: The maximum number of Arnoldi update iteration 
c                must be greater than zero.
c          = -5: WHICH must be one of 'LM', 'SM', 'LR', 'SR', 'LI', 'SI'
c          = -6: BMAT must be one of 'I' or 'G'.
c          = -7: Length of private work array is not sufficient.
c          = -8: Error return from LAPACK eigenvalue calculation;
c          = -9: Starting vector is zero.
c          = -10: IPARAM(7) must be 1,2,3.
c          = -11: IPARAM(7) = 1 and BMAT = 'G' are incompatable.
c          = -12: IPARAM(1) must be equal to 0 or 1.
c          = -9999: Could not build an Arnoldi factorization.
c                   User input error highly likely.  Please
c                   check actual array dimensions and layout.
c                   IPARAM(5) returns the size of the current Arnoldi
c                   factorization.
c  1. The computed Ritz values are approximate eigenvalues of OP. The
c     selection of WHICH should be made with this in mind when using
c     Mode = 3.  When operating in Mode = 3 setting WHICH = 'LM' will
c     compute the NEV eigenvalues of the original problem that are
c     closest to the shift SIGMA . After convergence, approximate eigenvalues 
c     of the original problem may be obtained with the ARPACK subroutine zneupd.
c  2. If a basis for the invariant subspace corresponding to the converged Ritz 
c     values is needed, the user must call zneupd immediately following 
c     completion of znaupd. This is new starting with release 2 of ARPACK.
c  3. If M can be factored into a Cholesky factorization M = LL'
c     then Mode = 2 should not be selected.  Instead one should use
c     Mode = 1 with  OP = inv(L)*A*inv(L').  Appropriate triangular 
c     linear systems should be solved with L and L' rather
c     than computing inverses.  After convergence, an approximate
c     eigenvector z of the original problem is recovered by solving
c     L'z = x  where x is a Ritz vector of OP.
c  4. At present there is no a-priori analysis to guide the selection of NCV 
c     relative to NEV.  The only formal requirement is that NCV > NEV + 2.
c     However, it is recommended that NCV .ge. 2*NEV+1.  If many problems of
c     the same type are to be solved, one should experiment with increasing
c     NCV while keeping NEV fixed for a given test problem.  This will 
c     usually decrease the required number of OP*x operations but it
c     also increases the work and storage required to maintain the orthogonal
c     basis vectors.  The optimal "cross-over" with respect to CPU time
c     is problem dependent and must be determined empirically. 
c     See Chapter 8 of Reference 2 for further information.
c  5. When IPARAM(1) = 0, and IDO = 3, the user needs to provide the
c     NP = IPARAM(8) complex shifts in locations
c     WORKL(IPNTR(14)), WORKL(IPNTR(14)+1), ... , WORKL(IPNTR(14)+NP).
c     Eigenvalues of the current upper Hessenberg matrix are located in
c     WORKL(IPNTR(6)) through WORKL(IPNTR(6)+NCV-1). They are ordered
c     according to the order defined by WHICH.  The associated Ritz estimates
c     are located in WORKL(IPNTR(8)), WORKL(IPNTR(8)+1), ... ,
c     WORKL(IPNTR(8)+NCV-1).

Chao Yang