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Packages » EliminationMatrices :: eliminationMatrix(List,Matrix,Matrix)
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# eliminationMatrix(List,Matrix,Matrix) -- returns a matrix corresponding to a residual resultant

- Function: eliminationMatrix
- Usage:
    `eliminationMatrix(v, r, m)`
- Inputs:
  - v, a list, list v of variables with respect to which the polynomials are homogeneous and from which one wants to remove these variables
  - r, a matrix, a single row matrix describing the base locus
  - m, a matrix, a matrix corresponding to the decomposition of a polynomial system over the base locus
- Optional inputs:
  - Strategy => ..., default value null, returns a matrix that represents the image of the map
- Outputs:
  - a matrix, a matrix corresponding to the residual resultant

## Description

If the strategy is 'CM2Residual':

Suppose given a homogeneous ideal locally complete intersection Cohen-Macaulay codimension 2 $J=(g_1,..,g_n)$, such that $I=(f_1,..,f_m)$ is included in J and (I:J) is a residual intersection. Let H be the matrix that I=J.H. Let R be the matrix of the first syzygies of J. This function computes an elimination matrix corresponding to the residual resultant over V(I) over V(J).

    i1 : R = QQ[X,Y,Z,x,y,z]

    o1 = R

    o1 : PolynomialRing

    i2 : F = matrix{{x*y^2,y^3,x*z^2,y^3+z^3}}

    o2 = | xy2 y3 xz2 y3+z3 |

                 1      4
    o2 : Matrix R  <-- R

    i3 : G = matrix{{y^2,z^2}}

    o3 = | y2 z2 |

                 1      2
    o3 : Matrix R  <-- R

    i4 : M = matrix{{1,0,0},{0,1,0},{0,0,1},{-X,-Y,-Z}}

    o4 = | 1  0  0  |
         | 0  1  0  |
         | 0  0  1  |
         | -X -Y -Z |

                 4      3
    o4 : Matrix R  <-- R

    i5 : H = (F//G)*M

    o5 = {2} | -Xy+x -Yy+y -Zy   |
         {2} | -Xz   -Yz   -Zz+x |

                 2      3
    o5 : Matrix R  <-- R

    i6 : l = {x,y,z}

    o6 = {x, y, z}

    o6 : List

    i7 : L=eliminationMatrix (l,G,H, Strategy => CM2Residual)

    o7 = {3} | 0  0   0  0  0  0  1  0  0  0    0    0    |
         {3} | 0  0   0  0  0  0  -X 1  0  -Y+1 0    0    |
         {3} | 0  0   -Y 0  0  0  -Z 0  1  0    0    0    |
         {3} | -1 0   0  0  0  0  0  -X 0  0    -Y+1 0    |
         {3} | 0  0   X  -Y 0  0  0  -Z -X -Z   0    -Y+1 |
         {3} | 0  0   0  0  -Y -1 0  0  -Z 0    0    0    |
         {3} | X  Y-1 0  0  0  Z  0  0  0  0    0    0    |
         {3} | 0  0   0  X  0  0  0  0  0  0    -Z   0    |
         {3} | 0  0   0  0  X  0  0  0  0  0    0    -Z   |
         {3} | X  Y   0  0  0  Z  0  0  0  0    0    0    |

                 10      12
    o7 : Matrix R   <-- R

    i8 : maxCol L

    o8 = {{3} | 0  0   0  0  0  0  1  0  0  0    |, {0, 1, 2, 3, 4, 5, 6, 7, 8,
          {3} | 0  0   0  0  0  0  -X 1  0  -Y+1 |
          {3} | 0  0   -Y 0  0  0  -Z 0  1  0    |
          {3} | -1 0   0  0  0  0  0  -X 0  0    |
          {3} | 0  0   X  -Y 0  0  0  -Z -X -Z   |
          {3} | 0  0   0  0  -Y -1 0  0  -Z 0    |
          {3} | X  Y-1 0  0  0  Z  0  0  0  0    |
          {3} | 0  0   0  X  0  0  0  0  0  0    |
          {3} | 0  0   0  0  X  0  0  0  0  0    |
          {3} | X  Y   0  0  0  Z  0  0  0  0    |
         ------------------------------------------------------------------------
         9}}

    o8 : List

If the strategy is 'ciResidual':

This function basically computes the matrix of the first application in the resolution of (I:J) given in the article of Bruns, Kustin and Miller: 'The resolution of the generic residual intersection of a complete intersection', Journal of Algebra 128.

The first argument is a list of homogeneous polynomials $J=(g_1,..,g_n)$forming a complete intersection with respect to the variables 'varList'. Given a system of homogeneous $I=(f_1,..,f_m)$, such that I is included in J and (I:J) is a residual intersection, one wants to compute a sort of resultant of (I:J). The second argument is the matrix M such that I=J.M. The output is a generically (with respect to the other variables than v) surjective matrix such that the determinant of a maximal minor is a multiple of the resultant of I on the closure of the complementary of V(J) in V(I). Such a minor can be obtain with maxMinor.

    i9 : R=QQ[a_0,a_1,a_2,a_3,a_4,b_0,b_1,b_2,b_3,b_4,c_0,c_1,c_2,c_3,c_4,x,y,z]

    o9 = R

    o9 : PolynomialRing

    i10 : G=matrix{{z,x^2+y^2}}

    o10 = | z x2+y2 |

                  1      2
    o10 : Matrix R  <-- R

    i11 : H=matrix{{a_0*z+a_1*x+a_2*y,b_0*z+b_1*x+b_2*y,c_0*z+c_1*x+c_2*y},{a_3,b_3,c_3}}

    o11 = | a_1x+a_2y+a_0z b_1x+b_2y+b_0z c_1x+c_2y+c_0z |
          | a_3            b_3            c_3            |

                  2      3
    o11 : Matrix R  <-- R

    i12 : L=eliminationMatrix ({x,y,z},G,H, Strategy => ciResidual)

    o12 = {2} | a_3 b_3 c_3 -a_3b_1+a_1b_3 0              0
          {2} | 0   0   0   -a_3b_2+a_2b_3 -a_3b_1+a_1b_3 0
          {2} | a_1 b_1 c_1 -a_3b_0+a_0b_3 0              -a_3b_1+a_1b_3
          {2} | a_3 b_3 c_3 0              -a_3b_2+a_2b_3 0
          {2} | a_2 b_2 c_2 0              -a_3b_0+a_0b_3 -a_3b_2+a_2b_3
          {2} | a_0 b_0 c_0 0              0              -a_3b_0+a_0b_3
          -----------------------------------------------------------------------
          -a_3c_1+a_1c_3 0              0              -b_3c_1+b_1c_3
          -a_3c_2+a_2c_3 -a_3c_1+a_1c_3 0              -b_3c_2+b_2c_3
          -a_3c_0+a_0c_3 0              -a_3c_1+a_1c_3 -b_3c_0+b_0c_3
          0              -a_3c_2+a_2c_3 0              0
          0              -a_3c_0+a_0c_3 -a_3c_2+a_2c_3 0
          0              0              -a_3c_0+a_0c_3 0
          -----------------------------------------------------------------------
          0              0              |
          -b_3c_1+b_1c_3 0              |
          0              -b_3c_1+b_1c_3 |
          -b_3c_2+b_2c_3 0              |
          -b_3c_0+b_0c_3 -b_3c_2+b_2c_3 |
          0              -b_3c_0+b_0c_3 |

                  6      12
    o12 : Matrix R  <-- R

If the strategy is 'byResolution':

This function computes the matrix of the first application in the resolution of (I:J) given by freeResolutionin degree regularity

    i13 : R=QQ[a_0,a_1,a_2,a_3,a_4,b_0,b_1,b_2,b_3,b_4,c_0,c_1,c_2,c_3,c_4,x,y,z]

    o13 = R

    o13 : PolynomialRing

    i14 : G=matrix{{z,x^2+y^2}}

    o14 = | z x2+y2 |

                  1      2
    o14 : Matrix R  <-- R

    i15 : H=matrix{{a_0*z+a_1*x+a_2*y,b_0*z+b_1*x+b_2*y,c_0*z+c_1*x+c_2*y},{a_3,b_3,c_3}}

    o15 = | a_1x+a_2y+a_0z b_1x+b_2y+b_0z c_1x+c_2y+c_0z |
          | a_3            b_3            c_3            |

                  2      3
    o15 : Matrix R  <-- R

    i16 : L=eliminationMatrix ({x,y,z},G,H, Strategy => byResolution)

    o16 = {2} | a_3b_1-a_1b_3 0             0             a_3c_1-a_1c_3
          {2} | a_3b_2-a_2b_3 a_3b_1-a_1b_3 0             a_3c_2-a_2c_3
          {2} | a_3b_0-a_0b_3 0             a_3b_1-a_1b_3 a_3c_0-a_0c_3
          {2} | 0             a_3b_2-a_2b_3 0             0
          {2} | 0             a_3b_0-a_0b_3 a_3b_2-a_2b_3 0
          {2} | 0             0             a_3b_0-a_0b_3 0
          -----------------------------------------------------------------------
          0             0             b_3c_1-b_1c_3 0             0
          a_3c_1-a_1c_3 0             b_3c_2-b_2c_3 b_3c_1-b_1c_3 0
          0             a_3c_1-a_1c_3 b_3c_0-b_0c_3 0             b_3c_1-b_1c_3
          a_3c_2-a_2c_3 0             0             b_3c_2-b_2c_3 0
          a_3c_0-a_0c_3 a_3c_2-a_2c_3 0             b_3c_0-b_0c_3 b_3c_2-b_2c_3
          0             a_3c_0-a_0c_3 0             0             b_3c_0-b_0c_3
          -----------------------------------------------------------------------
          a_3 b_3 c_3 |
          0   0   0   |
          a_1 b_1 c_1 |
          a_3 b_3 c_3 |
          a_2 b_2 c_2 |
          a_0 b_0 c_0 |

                  6      12
    o16 : Matrix R  <-- R

## See also

- macaulayFormula -- returns two matrices such that the ratio of their determinants is the Macaulay resultant
- bezoutianMatrix -- returns a matrix associated to generalized resultants

## Ways to use this method:

- eliminationMatrix(List,Matrix,Matrix) -- returns a matrix corresponding to a residual resultant

The source of this document is in /build/macaulay2-88fgJW/macaulay2-1.25.11+ds/M2/Macaulay2/packages/EliminationMatrices.m2:1356:0.