NAG Library Manual, Mark 28.4
```    Program f08fdfe

!     F08FDF Example Program Text

!     Mark 28.4 Release. NAG Copyright 2022.

!     .. Use Statements ..
Use nag_library, Only: dsyevr, nag_wp, x04caf
!     .. Implicit None Statement ..
Implicit None
!     .. Parameters ..
Real (Kind=nag_wp), Parameter    :: zero = 0.0E+0_nag_wp
Integer, Parameter               :: nb = 64, nin = 5, nout = 6
!     .. Local Scalars ..
Real (Kind=nag_wp)               :: abstol, vl, vu
Integer                          :: i, ifail, il, info, iu, lda, ldz,    &
liwork, lwork, m, n
!     .. Local Arrays ..
Real (Kind=nag_wp), Allocatable  :: a(:,:), w(:), work(:), z(:,:)
Real (Kind=nag_wp)               :: dummy(1)
Integer                          :: idum(1)
Integer, Allocatable             :: isuppz(:), iwork(:)
!     .. Intrinsic Procedures ..
Intrinsic                        :: max, nint
!     .. Executable Statements ..
Write (nout,*) 'F08FDF Example Program Results'
Write (nout,*)
!     Skip heading in data file and read N and the lower and upper
!     indices of the smallest and largest eigenvalues to be found
lda = n
ldz = n
m = n
Allocate (a(lda,n),w(n),z(ldz,m),isuppz(2*m))

!     Use routine workspace query to get optimal workspace.
lwork = -1
liwork = -1
!     The NAG name equivalent of dsyevr is f08fdf
Call dsyevr('Vectors','I','Upper',n,a,lda,vl,vu,il,iu,abstol,m,w,z,ldz,  &
isuppz,dummy,lwork,idum,liwork,info)

!     Make sure that there is enough workspace for block size nb.
lwork = max((nb+6)*n,nint(dummy(1)))
liwork = max(10*n,idum(1))
Allocate (work(lwork),iwork(liwork))

!     Read the upper triangular part of the matrix A from data file

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

abstol = zero

!     Solve the symmetric eigenvalue problem
!     The NAG name equivalent of dsyevr is f08fdf
Call dsyevr('Vectors','I','Upper',n,a,lda,vl,vu,il,iu,abstol,m,w,z,ldz,  &
isuppz,work,lwork,iwork,liwork,info)

If (info==0) Then

!       Print solution

Write (nout,*) 'Selected eigenvalues'
Write (nout,99999) 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)

Else
Write (nout,99998) 'Failure in DSYEVR. INFO =', info
End If

99999 Format (3X,(8F8.4))
99998 Format (1X,A,I5)
End Program f08fdfe
```