Program f07jbfe

!     F07JBF Example Program Text

!     Mark 27.3 Release. NAG Copyright 2021.

!     .. Use Statements ..
      Use nag_library, Only: dptsvx, nag_wp, x04caf
!     .. Implicit None Statement ..
      Implicit None
!     .. Parameters ..
      Integer, Parameter               :: nin = 5, nout = 6
!     .. Local Scalars ..
      Real (Kind=nag_wp)               :: rcond
      Integer                          :: i, ifail, info, ldb, ldx, n, nrhs
!     .. Local Arrays ..
      Real (Kind=nag_wp), Allocatable  :: b(:,:), berr(:), d(:), df(:), e(:),  &
                                          ef(:), ferr(:), work(:), x(:,:)
!     .. Executable Statements ..
      Write (nout,*) 'F07JBF Example Program Results'
      Write (nout,*)
      Flush (nout)
!     Skip heading in data file
      Read (nin,*)
      Read (nin,*) n, nrhs
      ldb = n
      ldx = n
      Allocate (b(ldb,nrhs),berr(nrhs),d(n),df(n),e(n-1),ef(n-1),ferr(nrhs),   &
        work(2*n),x(ldx,nrhs))

!     Read the lower bidiagonal part of the tridiagonal matrix A and
!     the right hand side b from data file

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

!     Solve the equations AX = B for X

!     The NAG name equivalent of dptsvx is f07jbf
      Call dptsvx('Not factored',n,nrhs,d,e,df,ef,b,ldb,x,ldx,rcond,ferr,berr, &
        work,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

        If (info==n+1) Then
          Write (nout,*)
          Write (nout,*) 'The matrix A is singular to working precision'
        End If
      Else
        Write (nout,99998) 'The leading minor of order ', info,                &
          ' is not positive definite'
      End If

99999 Format (1X,1P,7E11.1)
99998 Format (1X,A,I3,A)
    End Program f07jbfe