NAG Library Manual, Mark 28.6
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```    Program f11dxfe

!     F11DXF Example Program Text

!     Mark 28.6 Release. NAG Copyright 2022.

!     .. Use Statements ..
Use nag_library, Only: f11brf, f11bsf, f11btf, f11dxf, f11xnf, nag_wp
!     .. Implicit None Statement ..
Implicit None
!     .. Parameters ..
Integer, Parameter               :: nin = 5, nout = 6
!     .. Local Scalars ..
Real (Kind=nag_wp)               :: anorm, sigmax, stplhs, stprhs, tol
Integer                          :: i, ifail, ifail1, irevcm, iterm,     &
itn, lwork, lwreq, m, maxitn, monit, &
n, niter, nnz
Logical                          :: verbose
Character (1)                    :: init, norm, precon, weight
Character (8)                    :: method
!     .. Local Arrays ..
Complex (Kind=nag_wp), Allocatable :: a(:), b(:), diag(:), work(:), x(:)
Real (Kind=nag_wp), Allocatable  :: wgt(:)
Integer, Allocatable             :: icol(:), irow(:)
!     .. Intrinsic Procedures ..
Intrinsic                        :: log, nint
!     .. Executable Statements ..
Write (nout,*) 'F11DXF Example Program Results'

!     Skip heading in data file

lwork = 300
Allocate (a(nnz),b(n),diag(n),work(lwork),x(n),wgt(n),icol(nnz),         &
irow(nnz))

!     Read or initialize the parameters for the iterative solver

Read (nin,*) precon, norm, weight, iterm
Read (nin,*) m, tol, maxitn
anorm = 0.0E0_nag_wp
sigmax = 0.0E0_nag_wp

!     Read the parameters for the preconditioner

!     Read the nonzero elements of the matrix A

Do i = 1, nnz
Read (nin,*) a(i), irow(i), icol(i)
End Do

!     Read right-hand side vector b and initial approximate solution

!     Call F11BDF to initialize the solver

!     ifail: behaviour on error exit
!             =0 for hard exit, =1 for quiet-soft, =-1 for noisy-soft
ifail = 0
Call f11brf(method,precon,norm,weight,iterm,n,m,tol,maxitn,anorm,sigmax, &
monit,lwreq,work,lwork,ifail)

!     Call repeatedly F11BSF to solve the equations
!     Note that the arrays B and X are overwritten

!     On final exit, X will contain the solution and B the residual
!     vector

irevcm = 0
init = 'I'

ifail = 0
loop: Do
Call f11bsf(irevcm,x,b,wgt,work,lwreq,ifail)

If (irevcm/=4) Then
ifail1 = -1
If (irevcm==-1) Then
Call f11xnf('Transpose',n,nnz,a,irow,icol,'No checking',x,b,       &
ifail1)
Else If (irevcm==1) Then
Call f11xnf('No transpose',n,nnz,a,irow,icol,'No checking',x,b,    &
ifail1)
Else If (irevcm==2) Then
Call f11dxf('Non Hermitian','N',init,niter,n,nnz,a,irow,icol,      &
'Check',x,b,diag,work(lwreq+1),ifail1)
init = 'N'
Else If (irevcm==3) Then
Call f11btf(itn,stplhs,stprhs,anorm,sigmax,work,lwreq,ifail1)
If (ifail1==0) Then
If (itn<=3) Then
Write (nout,99999) itn
Write (nout,99998) nint(log(stplhs)/log(10.0_nag_wp))
End If
End If
End If
If (ifail1/=0) Then
irevcm = 6
End If
Else
Exit loop
End If
End Do loop

!     Obtain information about the computation

ifail1 = 0
Call f11btf(itn,stplhs,stprhs,anorm,sigmax,work,lwreq,ifail1)

!     Print the output data

Write (nout,99997)
verbose = .False.
If (verbose) Then
Write (nout,99996) 'Number of iterations for convergence:    ', itn
Write (nout,99995) 'Residual norm:                           ', stplhs
Write (nout,99995) 'Right-hand side of termination criterion:', stprhs
Write (nout,99995) '1-norm of matrix A:                      ', anorm
End If

!     Output x

Write (nout,99994)
Write (nout,99993) x(1:n)

99999 Format (/,1X,'Monitoring at iteration number',I4)
99998 Format (1X,'  order of residual norm:',I4)
99997 Format (/,1X,'Final Results')
99996 Format (1X,A,I5)
99995 Format (1X,A,1P,E11.1)
99994 Format (/,2X,'  Solution vector')
99993 Format (1X,'(',F8.3,',',F8.3,')')
End Program f11dxfe
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