Program f08ucfe

!     F08UCF Example Program Text

!     Mark 30.2 Release. NAG Copyright 2024.

!     .. Use Statements ..
      Use nag_library, Only: dsbgvd, nag_wp
!     .. Implicit None Statement ..
      Implicit None
!     .. Parameters ..
      Integer, Parameter               :: nin = 5, nout = 6
      Character (1), Parameter         :: uplo = 'U'
!     .. Local Scalars ..
      Integer                          :: i, info, j, ka, kb, ldab, ldbb,      &
                                          liwork, lwork, n
!     .. Local Arrays ..
      Real (Kind=nag_wp), Allocatable  :: ab(:,:), bb(:,:), w(:), work(:)
      Real (Kind=nag_wp)               :: dummy(1,1)
      Integer, Allocatable             :: iwork(:)
!     .. Intrinsic Procedures ..
      Intrinsic                        :: max, min
!     .. Executable Statements ..
      Write (nout,*) 'F08UCF Example Program Results'
      Write (nout,*)
!     Skip heading in data file
      Read (nin,*)
      Read (nin,*) n, ka, kb
      ldab = ka + 1
      ldbb = kb + 1
      liwork = 1
      lwork = 3*n
      Allocate (ab(ldab,n),bb(ldbb,n),w(n),work(lwork),iwork(liwork))

!     Read the upper or lower triangular parts of the matrices A and
!     B from data file

      If (uplo=='U') Then
        Read (nin,*)((ab(ka+1+i-j,j),j=i,min(n,i+ka)),i=1,n)
        Read (nin,*)((bb(kb+1+i-j,j),j=i,min(n,i+kb)),i=1,n)
      Else If (uplo=='L') Then
        Read (nin,*)((ab(1+i-j,j),j=max(1,i-ka),i),i=1,n)
        Read (nin,*)((bb(1+i-j,j),j=max(1,i-kb),i),i=1,n)
      End If

!     Solve the generalized symmetric band eigenvalue problem
!     A*x = lambda*B*x

!     The NAG name equivalent of dsbgvd is f08ucf
      Call dsbgvd('No vectors',uplo,n,ka,kb,ab,ldab,bb,ldbb,w,dummy,1,work,    &
        lwork,iwork,liwork,info)

      If (info==0) Then

!       Print solution

        Write (nout,*) 'Eigenvalues'
        Write (nout,99999) w(1:n)
      Else If (info>n .And. info<=2*n) Then
        i = info - n
        Write (nout,99998) 'The leading minor of order ', i,                   &
          ' of B is not positive definite'
      Else
        Write (nout,99997) 'Failure in DSBGVD. INFO =', info
      End If

99999 Format (3X,(6F11.4))
99998 Format (1X,A,I4,A)
99997 Format (1X,A,I4)
    End Program f08ucfe