linearlyReductiveAction -- Linearly reductive group action

Synopsis

• Usage:

  linearlyReductiveAction(I, M, R)

  linearlyReductiveAction(I, M, Q)

• Inputs:
  • I, an ideal, of a polynomial ring S defining a group as an affine variety
  • M, a matrix, whose entries are in S, that encodes the group action on R
  • R, a polynomial ring, on which the group acts
  • Q, a quotient ring, on which the group acts
• Outputs:
  • an instance of the type LinearlyReductiveAction, the linearly reductive action of S/I on R or Q via the matrix M

Description

In order to encode a linearly reductive group action, we represent the group as an affine variety. The polynomial ring S is the coordinate ring of the ambient affine space containing the group, while I is the ideal of S defining the group as a subvariety. In other words, the elements of the group are the points of the affine variety with coordinate ring S/I. The group acts linearly on the polynomial ring R via the matrix M with entries in S.

The next example constructs a cyclic group of order 2 as a set of two affine points. Then it introduces an action of this group on a polynomial ring in two variables.

i1 : S = QQ[z]

o1 = S

o1 : PolynomialRing

i2 : I = ideal(z^2 - 1)

            2
o2 = ideal(z  - 1)

o2 : Ideal of S

i3 : M = matrix{{(z+1)/2, (1-z)/2},{(1-z)/2, (z+1)/2}}

o3 = | 1/2z+1/2  -1/2z+1/2 |
     | -1/2z+1/2 1/2z+1/2  |

             2      2
o3 : Matrix S  <-- S

i4 : sub(M,z=>1),sub(M,z=>-1)

o4 = (| 1 0 |, | 0 1 |)
      | 0 1 |  | 1 0 |

o4 : Sequence

i5 : R = QQ[x,y]

o5 = R

o5 : PolynomialRing

i6 : L = linearlyReductiveAction(I, M, R)

                   2
o6 = R <- S/ideal(z  - 1) via 

     | 1/2z+1/2  -1/2z+1/2 |
     | -1/2z+1/2 1/2z+1/2  |

o6 : LinearlyReductiveAction

This function is also used to define linearly reductive group actions on quotients of polynomial rings. We illustrate by a slight variation on the previous example.

i7 : S = QQ[z];

i8 : I = ideal(z^2 - 1);

o8 : Ideal of S

i9 : M = matrix{{(z+1)/2, (1-z)/2},{(1-z)/2, (z+1)/2}};

             2      2
o9 : Matrix S  <-- S

i10 : Q = QQ[x,y] / ideal(x*y)

o10 = Q

o10 : QuotientRing

i11 : L = linearlyReductiveAction(I, M, Q)

                    2
o11 = Q <- S/ideal(z  - 1) via 

      | 1/2z+1/2  -1/2z+1/2 |
      | -1/2z+1/2 1/2z+1/2  |

o11 : LinearlyReductiveAction