PROGRAM f08gnfe

!      F08GNF Example Program Text

!      Mark 23 Release. NAG Copyright 2011.

!      .. Use Statements ..
       USE nag_library, ONLY : nag_wp, x02ajf, zhpev
!      .. Implicit None Statement ..
       IMPLICIT NONE
!      .. Parameters ..
       INTEGER, PARAMETER              :: nin = 5, nout = 6
       CHARACTER (1), PARAMETER        :: uplo = 'U'
!      .. Local Scalars ..
       REAL (KIND=nag_wp)              :: eerrbd, eps
       INTEGER                         :: i, info, j, n
!      .. Local Arrays ..
       COMPLEX (KIND=nag_wp), ALLOCATABLE :: ap(:), work(:)
       COMPLEX (KIND=nag_wp)           :: dummy(1,1)
       REAL (KIND=nag_wp), ALLOCATABLE :: rwork(:), w(:)
!      .. Intrinsic Functions ..
       INTRINSIC                          abs, max
!      .. Executable Statements ..
       WRITE (nout,*) 'F08GNF Example Program Results'
       WRITE (nout,*)
!      Skip heading in data file
       READ (nin,*)
       READ (nin,*) n

       ALLOCATE (ap((n*(n+1))/2),work(2*n-1),rwork(3*n-2),w(n))

!      Read the upper or lower triangular part of the matrix A from
!      data file

       IF (uplo=='U') THEN
          READ (nin,*) ((ap(i+(j*(j-1))/2),j=i,n),i=1,n)
       ELSE IF (uplo=='L') THEN
          READ (nin,*) ((ap(i+((2*n-j)*(j-1))/2),j=1,i),i=1,n)
       END IF

!      Solve the Hermitian eigenvalue problem

!      The NAG name equivalent of zhpev is f08gnf
       CALL zhpev('No vectors',uplo,n,ap,w,dummy,1,work,rwork,info)

       IF (info==0) THEN

!         Print solution

          WRITE (nout,*) 'Eigenvalues'
          WRITE (nout,99999) w(1:n)

!         Get the machine precision, EPS and compute the approximate
!         error bound for the computed eigenvalues.  Note that for
!         the 2-norm, max( abs(W(i)) ) = norm(A), and since the
!         eigenvalues are returned in ascending order
!         max( abs(W(i)) ) = max( abs(W(1)), abs(W(n)))

          eps = x02ajf()
          eerrbd = eps*max(abs(w(1)),abs(w(n)))

!         Print the approximate error bound for the eigenvalues

          WRITE (nout,*)
          WRITE (nout,*) 'Error estimate for the eigenvalues'
          WRITE (nout,99998) eerrbd
       ELSE
          WRITE (nout,99997) 'Failure in ZHPEV. INFO =', info
       END IF

99999  FORMAT (3X,(8F8.4))
99998  FORMAT (4X,1P,6E11.1)
99997  FORMAT (1X,A,I4)
    END PROGRAM f08gnfe