NAG Library Manual, Mark 27.2
```    Program f07jvfe

!     F07JVF Example Program Text

!     Mark 27.2 Release. NAG Copyright 2021.

!     .. Use Statements ..
Use nag_library, Only: nag_wp, x04dbf, zptrfs, zpttrf, zpttrs
!     .. Implicit None Statement ..
Implicit None
!     .. Parameters ..
Integer, Parameter               :: nin = 5, nout = 6
!     .. Local Scalars ..
Integer                          :: i, ifail, info, ldb, ldx, n, nrhs
!     .. Local Arrays ..
Complex (Kind=nag_wp), Allocatable :: b(:,:), e(:), ef(:), work(:),      &
x(:,:)
Real (Kind=nag_wp), Allocatable  :: berr(:), d(:), df(:), ferr(:),       &
rwork(:)
Character (1)                    :: clabs(1), rlabs(1)
!     .. Executable Statements ..
Write (nout,*) 'F07JVF Example Program Results'
Write (nout,*)
Flush (nout)
!     Skip heading in data file
ldb = n
ldx = n
Allocate (b(ldb,nrhs),e(n-1),ef(n-1),work(n),x(ldx,nrhs),berr(nrhs),     &
d(n),df(n),ferr(nrhs),rwork(n))

!     Read the lower bidiagonal part of the tridiagonal matrix A from
!     data file

!     Read the right hand matrix B

!     Copy A into DF and EF, and copy B into X
df(1:n) = d(1:n)
ef(1:n-1) = e(1:n-1)
x(1:n,1:nrhs) = b(1:n,1:nrhs)

!     Factorize the copy of the tridiagonal matrix A
!     The NAG name equivalent of zptrrf is f07jrf
Call zpttrf(n,df,ef,info)

If (info==0) Then

!       Solve the equations AX = B
!       The NAG name equivalent of zptrrs is f07jsf
Call zpttrs('Lower',n,nrhs,df,ef,x,ldx,info)

!       Improve the solution and compute error estimates
!       The NAG name equivalent of zptrfs is f07jvf
Call zptrfs('Lower',n,nrhs,d,e,df,ef,b,ldb,x,ldx,ferr,berr,work,rwork, &
info)

!       Print the solution and the forward and backward error estimates

!       ifail: behaviour on error exit
!              =0 for hard exit, =1 for quiet-soft, =-1 for noisy-soft
ifail = 0
Call x04dbf('General',' ',n,nrhs,x,ldx,'Bracketed','F7.4',             &
'Solution(s)','Integer',rlabs,'Integer',clabs,80,0,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)
Else
Write (nout,99998) 'The leading minor of order ', info,                &
' is not positive definite'
End If

99999 Format ((3X,1P,7E11.1))
99998 Format (1X,A,I3,A)
End Program f07jvfe
```