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

NAG FL Interface Introduction
Example description
    Program f07pgfe

!     F07PGF Example Program Text

!     Mark 28.5 Release. NAG Copyright 2022.

!     .. Use Statements ..
      Use nag_library, Only: dlansp => f06rdf, dspcon, dsptrf, nag_wp, x02ajf
!     .. Implicit None Statement ..
      Implicit None
!     .. Parameters ..
      Integer, Parameter               :: nin = 5, nout = 6
!     .. Local Scalars ..
      Real (Kind=nag_wp)               :: anorm, rcond
      Integer                          :: i, info, j, n
      Character (1)                    :: uplo
!     .. Local Arrays ..
      Real (Kind=nag_wp), Allocatable  :: ap(:), work(:)
      Integer, Allocatable             :: ipiv(:), iwork(:)
!     .. Executable Statements ..
      Write (nout,*) 'F07PGF Example Program Results'
!     Skip heading in data file
      Read (nin,*)
      Read (nin,*) n

      Allocate (ap(n*(n+1)/2),work(2*n),ipiv(n),iwork(n))

!     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

!     Compute norm of A
!     f06rdf is the NAG name equivalent of the LAPACK auxiliary dlansp
      anorm = dlansp('1-norm',uplo,n,ap,work)

!     Factorize A
!     The NAG name equivalent of dsptrf is f07pdf
      Call dsptrf(uplo,n,ap,ipiv,info)

      Write (nout,*)
      If (info==0) Then

!       Estimate condition number
!       The NAG name equivalent of dspcon is f07pgf
        Call dspcon(uplo,n,ap,ipiv,anorm,rcond,work,iwork,info)

        If (rcond>=x02ajf()) Then
          Write (nout,99999) 'Estimate of condition number =',                 &
          Write (nout,*) 'A is singular to working precision'
        End If
        Write (nout,*) 'The factor D is singular'
      End If

99999 Format (1X,A,1P,E10.2)
    End Program f07pgfe