NAG Library Manual, Mark 28.5
Interfaces:  FL   CL   CPP   AD 

NAG FL Interface Introduction
Example description
    Program f08jbfe

!     F08JBF Example Program Text

!     Mark 28.5 Release. NAG Copyright 2022.

!     .. Use Statements ..
      Use nag_library, Only: dstevx, nag_wp, x02amf, x04caf
!     .. Implicit None Statement ..
      Implicit None
!     .. Parameters ..
      Integer, Parameter               :: nin = 5, nout = 6
!     .. Local Scalars ..
      Real (Kind=nag_wp)               :: abstol, vl, vu
      Integer                          :: ifail, il, info, iu, ldz, m, n
!     .. Local Arrays ..
      Real (Kind=nag_wp), Allocatable  :: d(:), e(:), w(:), work(:), z(:,:)
      Integer, Allocatable             :: iwork(:), jfail(:)
!     .. Executable Statements ..
      Write (nout,*) 'F08JBF Example Program Results'
      Write (nout,*)
!     Skip heading in data file
      Read (nin,*)
      Read (nin,*) n
      ldz = n
      m = n
      Allocate (d(n),e(n),w(n),work(5*n),z(ldz,m),iwork(5*n),jfail(n))

!     Read the lower and upper bounds of the interval to be searched,
!     and read the diagonal and off-diagonal elements of the matrix
!     A from data file

      Read (nin,*) vl, vu
      Read (nin,*) d(1:n)
      Read (nin,*) e(1:n-1)

!     Set the absolute error tolerance for eigenvalues.  With ABSTOL
!     set to zero, the default value would be used instead

      abstol = 2.0E0_nag_wp*x02amf()

!     Solve the symmetric eigenvalue problem
!     The NAG name equivalent of dstevx is f08jbf
      Call dstevx('Vectors','Values in range',n,d,e,vl,vu,il,iu,abstol,m,w,z,  &

      If (info>=0) Then

!       Print solution

        Write (nout,99999) 'Number of eigenvalues found =', m
        Write (nout,*)
        Write (nout,*) 'Eigenvalues'
        Write (nout,99998) w(1:m)
        Flush (nout)

!       ifail: behaviour on error exit
!              =0 for hard exit, =1 for quiet-soft, =-1 for noisy-soft
        ifail = 0
        Call x04caf('General',' ',n,m,z,ldz,'Selected eigenvectors',ifail)

        If (info>0) Then
          Write (nout,99999) 'INFO eigenvectors failed to converge, INFO =',   &
          Write (nout,*) 'Indices of eigenvectors that did not converge'
          Write (nout,99997) jfail(1:m)
        End If
        Write (nout,99999) 'Failure in DSTEVX. INFO =', info
      End If

99999 Format (1X,A,I5)
99998 Format (3X,(8F8.4))
99997 Format (3X,(8I8))
    End Program f08jbfe