NAG Library Manual, Mark 27.2
```    Program f07bufe

!     F07BUF Example Program Text

!     Mark 27.2 Release. NAG Copyright 2021.

!     .. Use Statements ..
Use nag_library, Only: nag_wp, x02ajf, zgbcon, zgbtrf, zlangb => f06ubf
!     .. Implicit None Statement ..
Implicit None
!     .. Parameters ..
Integer, Parameter               :: nin = 5, nout = 6
Character (1), Parameter         :: norm = '1'
!     .. Local Scalars ..
Real (Kind=nag_wp)               :: anorm, rcond
Integer                          :: i, info, j, k, kl, ku, ldab, n
!     .. Local Arrays ..
Complex (Kind=nag_wp), Allocatable :: ab(:,:), work(:)
Real (Kind=nag_wp), Allocatable  :: rwork(:)
Integer, Allocatable             :: ipiv(:)
!     .. Intrinsic Procedures ..
Intrinsic                        :: max, min
!     .. Executable Statements ..
Write (nout,*) 'F07BUF Example Program Results'
!     Skip heading in data file
ldab = 2*kl + ku + 1
Allocate (ab(ldab,n),work(2*n),rwork(n),ipiv(n))

!     Read A from data file

k = kl + ku + 1

!     f06ubf is the NAG name equivalent of the LAPACK auxiliary zlangb
anorm = zlangb(norm,n,kl,ku,ab(kl+1,1),ldab,rwork)

!     Factorize A
!     The NAG name equivalent of zgbtrf is f07brf
Call zgbtrf(n,n,kl,ku,ab,ldab,ipiv,info)

Write (nout,*)
If (info==0) Then

!       Estimate condition number
!       The NAG name equivalent of zgbcon is f07buf
Call zgbcon(norm,n,kl,ku,ab,ldab,ipiv,anorm,rcond,work,rwork,info)

If (rcond>=x02ajf()) Then
Write (nout,99999) 'Estimate of condition number =',                 &
1.0E0_nag_wp/rcond
Else
Write (nout,*) 'A is singular to working precision'
End If
Else
Write (nout,*) 'The factor U is singular'
End If

99999 Format (1X,A,1P,E10.2)
End Program f07bufe
```