PROGRAM f08jafe ! F08JAF Example Program Text ! Mark 23 Release. NAG Copyright 2011. ! .. 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 Functions .. 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) ! Normalize the eigenvectors DO i = 1, n z(1:n,i) = z(1:n,i)/z(1,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