Program f08gtfe
! F08GTF Example Program Text
! Mark 30.0 Release. NAG Copyright 2024.
! .. Use Statements ..
Use nag_library, Only: dznrm2, nag_wp, x04dbf, zhptrd, zsteqr, zupgtr
! .. Implicit None Statement ..
Implicit None
! .. Parameters ..
Integer, Parameter :: nin = 5, nout = 6
! .. Local Scalars ..
Complex (Kind=nag_wp) :: scal
Integer :: i, ifail, info, j, k, ldq, n
Character (1) :: uplo
! .. Local Arrays ..
Complex (Kind=nag_wp), Allocatable :: ap(:), q(:,:), tau(:), work(:)
Real (Kind=nag_wp), Allocatable :: d(:), e(:), rwork(:)
Character (1) :: clabs(1), rlabs(1)
! .. Intrinsic Procedures ..
Intrinsic :: abs, conjg, maxloc
! .. Executable Statements ..
Write (nout,*) 'F08GTF Example Program Results'
! Skip heading in data file
Read (nin,*)
Read (nin,*) n
ldq = n
Allocate (ap(n*(n+1)/2),q(ldq,n),tau(n),work(n-1),d(n),e(n),rwork(2*n-2) &
)
! Read A from data file
Read (nin,*) uplo
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
! Reduce A to tridiagonal form T = (Q**H)*A*Q
! The NAG name equivalent of zhptrd is f08gsf
Call zhptrd(uplo,n,ap,d,e,tau,info)
! Form Q explicitly, storing the result in Q
! The NAG name equivalent of zupgtr is f08gtf
Call zupgtr(uplo,n,ap,tau,q,ldq,work,info)
! Calculate all the eigenvalues and eigenvectors of A
! The NAG name equivalent of zsteqr is f08jsf
Call zsteqr('V',n,d,e,q,ldq,rwork,info)
Write (nout,*)
If (info>0) Then
Write (nout,*) 'Failure to converge.'
Else
! Print eigenvalues and eigenvectors
Write (nout,*) 'Eigenvalues'
Write (nout,99999) d(1:n)
Write (nout,*)
Flush (nout)
! Normalize the eigenvectors, largest element real
Do i = 1, n
rwork(1:n) = abs(q(1:n,i))
k = maxloc(rwork(1:n),1)
scal = conjg(q(k,i))/abs(q(k,i))/dznrm2(n,q(1,i),1)
q(1:n,i) = q(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 x04dbf('General',' ',n,n,q,ldq,'Bracketed','F7.4','Eigenvectors', &
'Integer',rlabs,'Integer',clabs,80,0,ifail)
End If
99999 Format (8X,4(F7.4,11X,:))
End Program f08gtfe