Program f07mbfe

!     F07MBF Example Program Text

!     Mark 30.3 Release. nAG Copyright 2024.

!     .. Use Statements ..
      Use nag_library, Only: dsysvx, nag_wp, x04caf
!     .. Implicit None Statement ..
      Implicit None
!     .. Parameters ..
      Integer, Parameter               :: nb = 64, nin = 5, nout = 6
!     .. Local Scalars ..
      Real (Kind=nag_wp)               :: rcond
      Integer                          :: i, ifail, info, lda, ldaf, ldb, ldx, &
                                          lwork, n, nrhs
!     .. Local Arrays ..
      Real (Kind=nag_wp), Allocatable  :: a(:,:), af(:,:), b(:,:), berr(:),    &
                                          ferr(:), work(:), x(:,:)
      Integer, Allocatable             :: ipiv(:), iwork(:)
!     .. Executable Statements ..
      Write (nout,*) 'F07MBF Example Program Results'
      Write (nout,*)
      Flush (nout)
!     Skip heading in data file
      Read (nin,*)
      Read (nin,*) n, nrhs
      lda = n
      ldaf = n
      ldb = n
      ldx = n
      lwork = nb*n
      Allocate (a(lda,n),af(ldaf,n),b(ldb,nrhs),berr(nrhs),ferr(nrhs),         &
        work(lwork),x(ldx,nrhs),ipiv(n),iwork(n))

!     Read the upper triangular part of A from data file

      Read (nin,*)(a(i,i:n),i=1,n)

!     Read B from data file

      Read (nin,*)(b(i,1:nrhs),i=1,n)

!     Solve the equations AX = B for X
!     The NAG name equivalent of dsysvx is f07mbf
      Call dsysvx('Not factored','Upper',n,nrhs,a,lda,af,ldaf,ipiv,b,ldb,x,    &
        ldx,rcond,ferr,berr,work,lwork,iwork,info)

      If ((info==0) .Or. (info==n+1)) Then

!       Print solution, error bounds and condition number

!       ifail: behaviour on error exit
!              =0 for hard exit, =1 for quiet-soft, =-1 for noisy-soft
        ifail = 0
        Call x04caf('General',' ',n,nrhs,x,ldx,'Solution(s)',ifail)

        Write (nout,*)
        Write (nout,*) 'Backward errors (machine-dependent)'
        Write (nout,99999) berr(1:nrhs)
        Write (nout,*)
        Write (nout,*) 'Estimated forward error bounds (machine-dependent)'
        Write (nout,99999) ferr(1:nrhs)
        Write (nout,*)
        Write (nout,*) 'Estimate of reciprocal condition number'
        Write (nout,99999) rcond
        Write (nout,*)

        If (info==n+1) Then
          Write (nout,*)
          Write (nout,*) 'The matrix A is singular to working precision'
        End If
      Else
        Write (nout,99998) 'The diagonal block ', info, ' of D is zero'
      End If

99999 Format ((3X,1P,7E11.1))
99998 Format (1X,A,I3,A)
    End Program f07mbfe