Program f08gafe
! F08GAF Example Program Text
! Mark 28.7 Release. NAG Copyright 2022.
! .. Use Statements ..
Use nag_library, Only: dspev, nag_wp, x02ajf
! .. 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 ..
Real (Kind=nag_wp), Allocatable :: ap(:), w(:), work(:)
Real (Kind=nag_wp) :: dummy(1,1)
! .. Intrinsic Procedures ..
Intrinsic :: abs, max
! .. Executable Statements ..
Write (nout,*) 'F08GAF Example Program Results'
Write (nout,*)
! Skip heading in data file
Read (nin,*)
Read (nin,*) n
Allocate (ap((n*(n+1))/2),w(n),work(3*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 symmetric eigenvalue problem
! The NAG name equivalent of dspev is f08gaf
Call dspev('No vectors',uplo,n,ap,w,dummy,1,work,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 DSPEV. INFO =', info
End If
99999 Format (3X,(8F8.4))
99998 Format (4X,1P,6E11.1)
99997 Format (1X,A,I4)
End Program f08gafe