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= Elimination Matrices = = LU Decomposition =
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See [[LinearAlgebra/Elimination]] for a walkthrough of '''elimination'''. This regards the computation of '''elimination matrices''', which are a method of computing elimination. A fundamental step to solving systems of equation is [[LinearAlgebra/Elimination|elimination]]. A mathematical approach grounded in the same methods, but further generalized for automated compuation, is '''LU Decomposition'''.

<<TableOfContents>>

----
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== Introduction == == Linear Algebra ==
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The normal step of elimination proceeds like:

{{{
┌ ┐
│ [1] 2 1│
│ 3 8 1│
│ 0 4 1│
└ ┘
┌ ┐
│ [1] 2 1│
│ 0 2 -2│
│ 0 4 1│
└ ┘
}}}

Critically, note that this elimination step involved subtracting 3 of row 1 from row 2.

This can instead be formulated with matrices:

{{{
julia> A = [1 2 1; 3 8 1; 0 4 1]
3×3 Matrix{Int64}:
 1 2 1
 3 8 1
 0 4 1

julia> E2_1 = [1 0 0; -3 1 0; 0 0 1]
3×3 Matrix{Int64}:
  1 0 0
 -3 1 0
  0 0 1

julia> E2_1 * A
3×3 Matrix{Int64}:
 1 2 1
 0 2 -2
 0 4 1
}}}

This '''elimination matrix''' is called ''E,,2 1,,'' because it eliminated cell (2,1), the '''elimination cell'''. An elimination matrix is always the [[LinearAlgebra/SpecialMatrices#Identity_Matrix|identity matrix]] with the negated multiplier in the elimination cell.

Note that the next elimination step will involve subtracting 2 of row 2 from row 3, to the effect of eliminating cell (3,2). Therefore:

{{{
julia> E3_2 = [1 0 0; 0 1 0; 0 -2 1]
3×3 Matrix{Int64}:
 1 0 0
 0 1 0
 0 -2 1

julia> E3_2 * E2_1 * A
3×3 Matrix{Int64}:
 1 2 1
 0 2 -2
 0 0 5
}}}

----
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== Formulation ==
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The first step of elimination involves the elimination of the cell at row 2 column 1 ''(henceforward cell '''(2,1)''')''. == Simplification with Factorization ==

It is generally advantageous to refactor ''(E,,3 2,, E,,2 1,,) A = U'' into ''A = L U'', where ''L'' takes on the role of all elimination matrices. The notation ''L'' is a parallelism to the ''U'' notation for '''upper triangle matrices'''; ''L'' will be a '''lower triagle matrix'''.

''L'' can be trivially solved to be ''E,,2 1,,^-1^ E,,3 2,,^-1^''.

Furthermore, because elimination matrices are modifications of the [[LinearAlgebra/SpecialMatrices#Identity_Matrix|identity matrix]], their [[LinearAlgebra/MatrixInversion|inverses]] are also trivial to solve.
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[1] 2 1 [1] 2 1
 3 8 1 -> 0 2 -2
 0 4 1 0 4 1		             -1
  E E = I
   2,3 2,3
                  ┌ ┐
            -1 │ 1 0 0│
  E E = │ 0 1 0│
   2,3 2,3 │ 0 0 1│
                  └ ┘
┌ ┐ ┌ ┐
│ 1 0 0│ -1 │ 1 0 0│
│ -3 1 0│ E = │ 0 1 0│
│ 0 0 1│ 2,3 │ 0 0 1│
└ ┘ └ ┘
                  ┌ ┐
            -1 │ 1 0 0│
          E = │ 3 1 0│
           2,3 │ 0 0 1│
                  └ ┘
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This can instead be formulated in matrices: The fundamental reason for factorization is that the singular product of E,,3 2,, E,,2 1,, is messier than that of E,,2 1,,^-1^ E,,3 2,,^-1^
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┌ ┐┌ ┐ ┌ ┐
│ 1 0 0││ 1 2 1│ │ 1 2 1│
│ -3 1 0││ 3 8 1│ = │ 0 2 -2│
│ 0 0 1││ 0 4 1│ │ 0 4 1│
└ ┘└ ┘ └ ┘
   E E = ?
    3,2 2,1
┌ ┐┌ ┐ ┌ ┐
│ 1 0 0││ 1 0 0│ │ 1 0 0│
│ 0 1 0││ -3 1 0│ = │ -3 1 0│
│ 0 -2 1││ 0 0 1│ │ 6 -2 1│
└ ┘└ ┘ └ ┘

   -1 -1
  E E = L
   2,1 3,2
┌ ┐┌ ┐ ┌ ┐
│ 1 0 0││ 1 0 0│ │ 1 0 0│
│ 3 1 0││ 0 1 0│ = │ 3 1 0│
│ 0 0 1││ 0 2 1│ │ 0 2 1│
└ ┘└ ┘ └ ┘
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This elimination matrix is called E,,2 1,, because is eliminated cell (2,1). An elimination matrix is always the identity matric with the negative of the multiplier in the elimination position. In particular, cell (3,1) is a 0 in L but is 6 in E,,3 2,, E,,2 1,,.
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The full elimination process can be formulated as E,,3 2,, (E,,2 1,, A) = U. This is equivalent to (E,,3 2,, E,,2 1,,) A = U. ----



== Permutation ==

If a zero pivot is reached, rows must be exchanged. This can be expressed with matrices as P A = L U.

See [[LinearAlgebra/PermutationMatrices|Permutation Matrices]] for more information on these matrices and their properties.

LU Decomposition

A fundamental step to solving systems of equation is elimination. A mathematical approach grounded in the same methods, but further generalized for automated compuation, is LU Decomposition.

Contents

  1. LU Decomposition
    1. Linear Algebra
    2. Simplification with Factorization
    3. Permutation

Linear Algebra

Consider the below system of equations: 

x + 2y + z = 2
3x + 8y + z = 12
4y + z = 2

The normal step of elimination proceeds like: 

┌        ┐
│ [1] 2 1│
│  3  8 1│
│  0  4 1│
└        ┘
┌         ┐
│ [1] 2  1│
│  0  2 -2│
│  0  4  1│
└         ┘

Critically, note that this elimination step involved subtracting 3 of row 1 from row 2.

This can instead be formulated with matrices: 

julia> A = [1 2 1; 3 8 1; 0 4 1]
3×3 Matrix{Int64}:
 1  2  1
 3  8  1
 0  4  1

julia> E2_1 = [1 0 0; -3 1 0; 0 0 1]
3×3 Matrix{Int64}:
  1  0  0
 -3  1  0
  0  0  1

julia> E2_1 * A
3×3 Matrix{Int64}:
 1  2   1
 0  2  -2
 0  4   1

This elimination matrix is called E2 1 because it eliminated cell (2,1), the elimination cell. An elimination matrix is always the identity matrix with the negated multiplier in the elimination cell.

Note that the next elimination step will involve subtracting 2 of row 2 from row 3, to the effect of eliminating cell (3,2). Therefore: 

julia> E3_2 = [1 0 0; 0 1 0; 0 -2 1]
3×3 Matrix{Int64}:
 1   0  0
 0   1  0
 0  -2  1

julia> E3_2 * E2_1 * A
3×3 Matrix{Int64}:
 1  2   1
 0  2  -2
 0  0   5

Simplification with Factorization

It is generally advantageous to refactor (E3 2 E2 1) A = U into A = L U, where L takes on the role of all elimination matrices. The notation L is a parallelism to the U notation for upper triangle matrices; L will be a lower triagle matrix.

L can be trivially solved to be E2 1-1 E3 2-1.

Furthermore, because elimination matrices are modifications of the identity matrix, their inverses are also trivial to solve. 

            -1
  E       E     =     I
   2,3     2,3
                  ┌      ┐
            -1    │ 1 0 0│
  E       E     = │ 0 1 0│
   2,3     2,3    │ 0 0 1│
                  └      ┘
┌       ┐         ┌      ┐
│  1 0 0│   -1    │ 1 0 0│
│ -3 1 0│ E     = │ 0 1 0│
│  0 0 1│  2,3    │ 0 0 1│
└       ┘         └      ┘
                  ┌      ┐
            -1    │ 1 0 0│
          E     = │ 3 1 0│
           2,3    │ 0 0 1│
                  └      ┘

The fundamental reason for factorization is that the singular product of E3 2 E2 1 is messier than that of E2 1-1 E3 2-1 

   E        E      =      ?
    3,2      2,1
┌       ┐┌       ┐   ┌        ┐
│ 1  0 0││  1 0 0│   │  1  0 0│
│ 0  1 0││ -3 1 0│ = │ -3  1 0│
│ 0 -2 1││  0 0 1│   │  6 -2 1│
└       ┘└       ┘   └        ┘

   -1      -1
  E       E      =     L
   2,1     3,2    
┌      ┐┌      ┐   ┌      ┐
│ 1 0 0││ 1 0 0│   │ 1 0 0│
│ 3 1 0││ 0 1 0│ = │ 3 1 0│
│ 0 0 1││ 0 2 1│   │ 0 2 1│
└      ┘└      ┘   └      ┘

In particular, cell (3,1) is a 0 in L but is 6 in E3 2 E2 1.

Permutation

If a zero pivot is reached, rows must be exchanged. This can be expressed with matrices as P A = L U.

See Permutation Matrices for more information on these matrices and their properties.

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LinearAlgebra/LUDecomposition (last edited 2024-01-29 03:13:41 by DominicRicottone)