NAG Library Manual, Mark 28.7
```    Program f07cgfe

!     F07CGF Example Program Text

!     Mark 28.7 Release. NAG Copyright 2022.

!     .. Use Statements ..
Use nag_library, Only: dgtcon, dgttrf, f06rnf, nag_wp, x02ajf
!     .. Implicit None Statement ..
Implicit None
!     .. Parameters ..
Integer, Parameter               :: nin = 5, nout = 6
!     .. Local Scalars ..
Real (Kind=nag_wp)               :: anorm, rcond
Integer                          :: info, n
!     .. Local Arrays ..
Real (Kind=nag_wp), Allocatable  :: d(:), dl(:), du(:), du2(:), work(:)
Integer, Allocatable             :: ipiv(:), iwork(:)
!     .. Executable Statements ..
Write (nout,*) 'F07CGF Example Program Results'
Write (nout,*)
!     Skip heading in data file

Allocate (d(n),dl(n-1),du(n-1),du2(n-2),work(2*n),ipiv(n),iwork(n))

!     Read the tridiagonal matrix A from data file

!     Compute the 1-norm of A
anorm = f06rnf('1-norm',n,dl,d,du)

!     Factorize the tridiagonal matrix A
!     The NAG name equivalent of dgttrf is f07cdf
Call dgttrf(n,dl,d,du,du2,ipiv,info)

If (info==0) Then

!       Estimate the condition number of A
!       The NAG name equivalent of dgtcon is f07cgf
Call dgtcon('1-norm',n,dl,d,du,du2,ipiv,anorm,rcond,work,iwork,info)

!       Print the estimated condition number

If (rcond>=x02ajf()) Then
Write (nout,99999) 'Estimate of condition number = ',                &
1.0_nag_wp/rcond
Else
Write (nout,99999) 'A is singular to working precision. RCOND = ',   &
rcond
End If

Else
Write (nout,99998) 'The (', info, ',', info, ')',                      &
' element of the factor U is zero'
End If

99999 Format (1X,A,1P,E10.2)
99998 Format (1X,A,I3,A,I3,A,A)
End Program f07cgfe
```