Program f08gffe
! F08GFF Example Program Text
! Mark 30.0 Release. NAG Copyright 2024.
! .. Use Statements ..
Use nag_library, Only: blas_damax_val, dopgtr, dsptrd, dsteqr, nag_wp, &
x04caf
! .. Implicit None Statement ..
Implicit None
! .. Parameters ..
Real (Kind=nag_wp), Parameter :: zero = 0.0E0_nag_wp
Integer, Parameter :: nin = 5, nout = 6
! .. Local Scalars ..
Real (Kind=nag_wp) :: r
Integer :: i, ifail, info, j, k, ldq, n
Character (1) :: uplo
! .. Local Arrays ..
Real (Kind=nag_wp), Allocatable :: ap(:), d(:), e(:), q(:,:), tau(:), &
work(:)
! .. Executable Statements ..
Write (nout,*) 'F08GFF Example Program Results'
! Skip heading in data file
Read (nin,*)
Read (nin,*) n
ldq = n
Allocate (ap(n*(n+1)/2),d(n),e(n),q(ldq,n),tau(n),work(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**T)*A*Q
! The NAG name equivalent of dsptrd is f08gef
Call dsptrd(uplo,n,ap,d,e,tau,info)
! Form Q explicitly, storing the result in Q
! The NAG name equivalent of dopgtr is f08gff
Call dopgtr(uplo,n,ap,tau,q,ldq,work,info)
! Calculate all the eigenvalues and eigenvectors of A
! The NAG name equivalent of dsteqr is f08jef
Call dsteqr('V',n,d,e,q,ldq,work,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 positive
Do i = 1, n
Call blas_damax_val(n,q(1,i),1,k,r)
If (q(k,i)<zero) Then
q(1:n,i) = -q(1:n,i)
End If
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,q,ldq,'Eigenvectors',ifail)
End If
99999 Format (3X,(8F8.4))
End Program f08gffe