Program f07chfe ! F07CHF Example Program Text ! Mark 24 Release. NAG Copyright 2012. ! .. Use Statements .. Use nag_library, Only: dgtrfs, dgttrf, dgttrs, nag_wp, x04caf ! .. Implicit None Statement .. Implicit None ! .. Parameters .. Integer, Parameter :: nin = 5, nout = 6 ! .. Local Scalars .. Integer :: i, ifail, info, ldb, ldx, n, nrhs ! .. Local Arrays .. Real (Kind=nag_wp), Allocatable :: b(:,:), berr(:), d(:), df(:), dl(:), & dlf(:), du(:), du2(:), duf(:), & ferr(:), work(:), x(:,:) Integer, Allocatable :: ipiv(:), iwork(:) ! .. Executable Statements .. Write (nout,*) 'F07CHF 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),berr(nrhs),d(n),df(n),dl(n-1),dlf(n-1),du(n-1), & du2(n-2),duf(n-1),ferr(nrhs),work(3*n),x(ldx,nrhs),ipiv(n),iwork(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 dgttrf is f07cdf Call dgttrf(n,dlf,df,duf,du2,ipiv,info) If (info==0) Then ! Solve the equations AX = B ! The NAG name equivalent of dgttrs is f07cef Call dgttrs('No transpose',n,nrhs,dlf,df,duf,du2,ipiv,x,ldx,info) ! Improve the solution and compute error estimates ! The NAG name equivalent of dgtrfs is f07chf Call dgtrfs('No transpose',n,nrhs,dl,d,du,dlf,df,duf,du2,ipiv,b,ldb,x, & ldx,ferr,berr,work,iwork,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 x04caf('General',' ',n,nrhs,x,ldx,'Solution(s)',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 f07chfe