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Lapack95 example 5

This example shows the use of SymGetLDL method defined in Lapack95.

program main
use easifemBase
implicit none
real(dfp), allocatable ::  A(:,:), E(:), D(:), exactL(:,:), exactD(:)
real(dfp) :: error
integer(i4b), allocatable :: ipiv(:)
integer(i4b) :: info
character(len=1) :: uplo

allocate( A(3,3), D(3), E(3), ipiv(3), exactD(3), exactL(3,3))
A(1, :) = [2, -1, 3]
A(2, :) = [-1, 2, 0]
A(3, :) = [3, 0, 1]
CALL Display( A, "A = " )

exactL(:,1) = [1., -0.5, 1.5]
exactL(:,2) = [0., 1., 1.]
exactL(:,3) = [0., 0., 1.]
exactD = [2., 1.5, -5.0]

uplo = "U"

CALL SymGetLDL(A=A, D=D, E=E, UPLO=uplo, IPIV=ipiv, info=info)

CALL Display( exactL, "exact L = " )
CALL Display( exactD, "exact D = " )
CALL Display( A, "L, permuted = " )
CALL Display( D, "D, permuted = " )
CALL Display( E, "E, permuted " )
CALL Display( ipiv, "permutation = " )
CALL Display( info, "info = " )

CALL Display(MATMUL( MATMUL( A, Diag(D) ), TRANSPOSE(A)), &
  & "A = ")

end program main

A matrix is given by

            A =
----------------------------
 2.00000  -1.00000   3.00000
-1.00000   2.00000   0.00000
 3.00000   0.00000   1.00000

exact L is given by:

         exact L =
----------------------------
 1.00000   0.00000   0.00000
-0.50000   1.00000   0.00000
 1.50000   1.00000   1.00000

Computed L L~\tilde{L} is given below.

       L, permuted =
----------------------------
 0.00000   0.00000   1.00000
 0.00000   1.00000  -0.50000
 1.00000   1.00000   1.50000

We can see that col 1 and col3 needs to interchange to get the exact L.

exact D is given by:

exact D =
----------
  2.00000
  1.50000
 -5.00000

Computed D, D~\tilde{D} is given below:

D, permuted =
--------------
   -5.00000
    1.50000
    2.00000

The permutation is given by

permutation =
--------------
      3
      2
      1

We can see that D(1) and D(3) should be interchange to get the exact D.

If we perform

L~⋅D~⋅L~T\tilde{L} \cdot \tilde{D} \cdot \tilde{L}^{T}

We get

L⋅D⋅LTL \cdot D \cdot L^{T} 
            A =
----------------------------
 2.00000  -1.00000   3.00000
-1.00000   2.00000   0.00000
 3.00000   0.00000   1.00000

If you want to calculate exact D from computed D, please call LAPMT routine and pass the ipiv returned by the routine.