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

NAG FL Interface Introduction
Example description
    Program f07fgfe

!     F07FGF Example Program Text

!     Mark 28.5 Release. NAG Copyright 2022.

!     .. Use Statements ..
      Use nag_library, Only: dlansy => f06rcf, dpocon, dpotrf, 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, lda, n
      Character (1)                    :: uplo
!     .. Local Arrays ..
      Real (Kind=nag_wp), Allocatable  :: a(:,:), work(:)
      Integer, Allocatable             :: iwork(:)
!     .. Executable Statements ..
      Write (nout,*) 'F07FGF Example Program Results'
!     Skip heading in data file
      Read (nin,*)
      Read (nin,*) n
      lda = n
      Allocate (a(lda,n),work(3*n),iwork(n))

!     Read A from data file

      Read (nin,*) uplo
      If (uplo=='U') Then
        Read (nin,*)(a(i,i:n),i=1,n)
      Else If (uplo=='L') Then
        Read (nin,*)(a(i,1:i),i=1,n)
      End If

!     Compute norm of A
!     f06rcf is the NAG name equivalent of the LAPACK auxiliary dlansy
      anorm = dlansy('1-norm',uplo,n,a,lda,work)

!     Factorize A
!     The NAG name equivalent of dpotrf is f06fdf
      Call dpotrf(uplo,n,a,lda,info)

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

!       Estimate condition number

!       The NAG name equivalent of dpocon is f07fgf
        Call dpocon(uplo,n,a,lda,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,*) 'A is not positive definite'
      End If

99999 Format (1X,A,1P,E10.2)
    End Program f07fgfe