NAG Library Manual, Mark 29
```!   G02HBF Example Program Text
!   Mark 29.0 Release. NAG Copyright 2023.

Module g02hbfe_mod

!     G02HBF Example Program Module:
!            Parameters and User-defined Routines

!     .. Use Statements ..
Use nag_library, Only: nag_wp
!     .. Implicit None Statement ..
Implicit None
!     .. Accessibility Statements ..
Private
Public                           :: ucv
!     .. Parameters ..
Real (Kind=nag_wp), Parameter, Public :: one = 1.0_nag_wp
Real (Kind=nag_wp), Parameter    :: two = 2.0_nag_wp
Real (Kind=nag_wp), Parameter    :: zero = 0.0_nag_wp
Integer, Parameter, Public       :: iset = 1, nin = 5, nout = 6
Contains
Function ucv(t)
!       UCV function for Krasker-Welsch weights

!       .. Use Statements ..
Use nag_library, Only: s15abf, x01aaf, x02akf
!       .. Function Return Value ..
Real (Kind=nag_wp)             :: ucv
!       .. Parameters ..
Real (Kind=nag_wp), Parameter  :: ucvc = 2.5_nag_wp
!       .. Scalar Arguments ..
Real (Kind=nag_wp), Intent (In) :: t
!       .. Local Scalars ..
Real (Kind=nag_wp)             :: pc, pd, q, q2
Integer                        :: ifail
!       .. Intrinsic Procedures ..
Intrinsic                      :: exp, log, sqrt
!       .. Executable Statements ..
ucv = one
If (t/=zero) Then
q = ucvc/t
q2 = q*q
ifail = 0
pc = s15abf(q,ifail)
If (q2<-log(x02akf())) Then
pd = exp(-q2/two)/sqrt(x01aaf(zero)*two)
Else
pd = zero
End If
ucv = (two*pc-one)*(one-q2) + q2 - two*q*pd
End If
Return
End Function ucv
End Module g02hbfe_mod
Program g02hbfe

!     G02HBF Example Main Program

!     .. Use Statements ..
Use g02hbfe_mod, Only: iset, nin, nout, one, ucv
Use nag_library, Only: g02hbf, nag_wp, x04abf, x04ccf
!     .. Implicit None Statement ..
Implicit None
!     .. Local Scalars ..
Real (Kind=nag_wp)               :: bd, bl, tol
Integer                          :: i, ifail, la, ldx, m, maxit, n,      &
!     .. Local Arrays ..
Real (Kind=nag_wp), Allocatable  :: a(:), wk(:), x(:,:), z(:)
!     .. Executable Statements ..
Write (nout,*) 'G02HBF Example Program Results'
Write (nout,*)
Flush (nout)

!     Skip heading in data file

!     Read in the problem size

ldx = n
la = (m+1)*m/2
Allocate (x(ldx,m),a(la),wk(la),z(n))

!     Read in initial value of A

Read (nin,*) nitmon, bl, bd, maxit, tol

!     Set the advisory channel to NOUT for monitoring information
If (nitmon/=0) Then
End If

!     Calculate A
ifail = 0
Call g02hbf(ucv,n,m,x,ldx,a,z,bl,bd,tol,maxit,nitmon,nit,wk,ifail)

!     Display results
Write (nout,99999) 'G02HBF required ', nit, ' iterations to converge'
Write (nout,*)
Flush (nout)
ifail = 0
Call x04ccf('Lower','Non-Unit',m,a,'Matrix A',ifail)
Write (nout,*)
Write (nout,*) 'Vector Z'
Write (nout,99998)(z(i),i=1,n)
Write (nout,*)
Write (nout,*) 'Vector of Krasker-Welsch weights'
Write (nout,99998)(one/z(i),i=1,n)

99999 Format (1X,A,I0,A)
99998 Format (1X,F9.4)
End Program g02hbfe
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