Program f08hnfe

!     F08HNF Example Program Text

!     Mark 29.2 Release. NAG Copyright 2023.

!     .. Use Statements ..
      Use nag_library, Only: ddisna, dznrm2, nag_wp, x02ajf, x04daf, zhbev
!     .. Implicit None Statement ..
      Implicit None
!     .. Parameters ..
      Integer, Parameter               :: nin = 5, nout = 6
      Character (1), Parameter         :: uplo = 'U'
!     .. Local Scalars ..
      Complex (Kind=nag_wp)            :: scal
      Real (Kind=nag_wp)               :: eerrbd, eps
      Integer                          :: i, ifail, info, j, k, kd, ldab, ldz, &
                                          n
!     .. Local Arrays ..
      Complex (Kind=nag_wp), Allocatable :: ab(:,:), work(:), z(:,:)
      Real (Kind=nag_wp), Allocatable  :: rcondz(:), rwork(:), w(:), zerrbd(:)
!     .. Intrinsic Procedures ..
      Intrinsic                        :: abs, conjg, max, maxloc, min
!     .. Executable Statements ..
      Write (nout,*) 'F08HNF Example Program Results'
      Write (nout,*)
!     Skip heading in data file
      Read (nin,*)
      Read (nin,*) n, kd
      ldab = kd + 1
      ldz = n
      Allocate (ab(ldab,n),work(n),z(ldz,n),rcondz(n),rwork(3*n-2),w(n),       &
        zerrbd(n))

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

      If (uplo=='U') Then
        Read (nin,*)((ab(kd+1+i-j,j),j=i,min(n,i+kd)),i=1,n)
      Else If (uplo=='L') Then
        Read (nin,*)((ab(1+i-j,j),j=max(1,i-kd),i),i=1,n)
      End If

!     Solve the band Hermitian eigenvalue problem
!     The NAG name equivalent of zhbev is f08hnf
      Call zhbev('Vectors',uplo,n,kd,ab,ldab,w,z,ldz,work,rwork,info)

      If (info==0) Then

!       Print solution

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

!       Normalize the eigenvectors, largest element real
        Do i = 1, n
          rwork(1:n) = abs(z(1:n,i))
          k = maxloc(rwork(1:n),1)
          scal = conjg(z(k,i))/abs(z(k,i))/dznrm2(n,z(1,i),1)
          z(1:n,i) = z(1:n,i)*scal
        End Do

!       ifail: behaviour on error exit
!              =0 for hard exit, =1 for quiet-soft, =-1 for noisy-soft
        ifail = 0
        Call x04daf('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(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)))

!       Call DDISNA (F08FLF) to estimate reciprocal condition
!       numbers for the eigenvectors
        Call ddisna('Eigenvectors',n,n,w,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 ZHBEV. INFO =', info
      End If

99999 Format (3X,(8F8.4))
99998 Format (4X,1P,6E11.1)
99997 Format (1X,A,I4)
    End Program f08hnfe