Program f07cvfe
! F07CVF Example Program Text
! Mark 30.1 Release. NAG Copyright 2024.
! .. Use Statements ..
Use nag_library, Only: nag_wp, x04dbf, zgtrfs, zgttrf, zgttrs
! .. 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(:,:), d(:), df(:), dl(:), &
dlf(:), du(:), du2(:), duf(:), &
work(:), x(:,:)
Real (Kind=nag_wp), Allocatable :: berr(:), ferr(:), rwork(:)
Integer, Allocatable :: ipiv(:)
Character (1) :: clabs(1), rlabs(1)
! .. Executable Statements ..
Write (nout,*) 'F07CVF 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),d(n),df(n),dl(n-1),dlf(n-1),du(n-1),du2(n-2), &
duf(n-1),work(2*n),x(ldx,nrhs),berr(nrhs),ferr(nrhs),rwork(n),ipiv(n))
! Read the tridiagonal matrix A from data file
Read (nin,*) du(1:n-1)
Read (nin,*) d(1:n)
Read (nin,*) dl(1:n-1)
! Read the right hand matrix B
Read (nin,*)(b(i,1:nrhs),i=1,n)
! Copy A into DUF, DF and DLF, and copy B into X
duf(1:n-1) = du(1:n-1)
df(1:n) = d(1:n)
dlf(1:n-1) = dl(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 zgttrf is f07crf
Call zgttrf(n,dlf,df,duf,du2,ipiv,info)
If (info==0) Then
! Solve the equations AX = B
! The NAG name equivalent of zgttrs is f07csf
Call zgttrs('No transpose',n,nrhs,dlf,df,duf,du2,ipiv,x,ldx,info)
! Improve the solution and compute error estimates
! The NAG name equivalent of zgtrfs is f07cvf
Call zgtrfs('No transpose',n,nrhs,dl,d,du,dlf,df,duf,du2,ipiv,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 (', info, ',', info, ')', &
' element of the factor U is zero'
End If
99999 Format ((3X,1P,7E11.1))
99998 Format (1X,A,I3,A,I3,A,A)
End Program f07cvfe