Program f08jafe ! F08JAF Example Program Text ! Mark 24 Release. NAG Copyright 2012. ! .. Use Statements .. Use nag_library, Only: ddisna, dstev, nag_wp, x02ajf, x04caf ! .. Implicit None Statement .. Implicit None ! .. Parameters .. Integer, Parameter :: nin = 5, nout = 6 ! .. Local Scalars .. Real (Kind=nag_wp) :: eerrbd, eps Integer :: i, ifail, info, ldz, n ! .. Local Arrays .. Real (Kind=nag_wp), Allocatable :: d(:), e(:), rcondz(:), work(:), & z(:,:), zerrbd(:) ! .. Intrinsic Procedures .. Intrinsic :: abs, max ! .. Executable Statements .. Write (nout,*) 'F08JAF Example Program Results' Write (nout,*) ! Skip heading in data file Read (nin,*) Read (nin,*) n ldz = n Allocate (d(n),e(n-1),rcondz(n),work(2*n-2),z(ldz,n),zerrbd(n)) ! Read the diagonal and off-diagonal elements of the matrix A ! from data file Read (nin,*) d(1:n) Read (nin,*) e(1:n-1) ! Solve the symmetric tridiagonal eigenvalue problem ! The NAG name equivalent of dstev is f08jaf Call dstev('Vectors',n,d,e,z,ldz,work,info) If (info==0) Then ! Print solution Write (nout,*) 'Eigenvalues' Write (nout,99999) d(1:n) Flush (nout) ! Standardize the eigenvectors so that first elements are non-negative. Do i = 1, n If (z(1,i)<0.0_nag_wp) z(1:n,i) = -z(1:n,i) End Do ! ifail: behaviour on error exit ! =0 for hard exit, =1 for quiet-soft, =-1 for noisy-soft ifail = 0 Call x04caf('General',' ',n,n,z,ldz,'Eigenvectors',ifail) ! Get the machine precision, EPS and compute the approximate ! error bound for the computed eigenvalues. Note that for ! the 2-norm, max( abs(D(i)) ) = norm(A), and since the ! eigenvalues are returned in ascending order ! max( abs(D(i)) ) = max( abs(D(1)), abs(D(n))) eps = x02ajf() eerrbd = eps*max(abs(d(1)),abs(d(n))) ! Call DDISNA (F08FLF) to estimate reciprocal condition ! numbers for the eigenvectors Call ddisna('Eigenvectors',n,n,d,rcondz,info) ! Compute the error estimates for the eigenvectors Do i = 1, n zerrbd(i) = eerrbd/rcondz(i) End Do ! Print the approximate error bounds for the eigenvalues ! and vectors Write (nout,*) Write (nout,*) 'Error estimate for the eigenvalues' Write (nout,99998) eerrbd Write (nout,*) Write (nout,*) 'Error estimates for the eigenvectors' Write (nout,99998) zerrbd(1:n) Else Write (nout,99997) 'Failure in DSTEV. INFO =', info End If 99999 Format (3X,(8F8.4)) 99998 Format (4X,1P,6E11.1) 99997 Format (1X,A,I4) End Program f08jafe