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NormalToricVarieties :: orbits(NormalToricVariety,ZZ)

orbits(NormalToricVariety,ZZ) -- get a list of the torus orbits of a given dimension

Synopsis

Description

A normal toric variety is a disjoint union of its orbits under the action of its algebraic torus. These orbits are in bijection with the cones in the associated fan. Each cone is determined by the rays it contains. In this package, the rays are ordered and indexed by nonnegative integers: 0, 1, …, n. Using this indexing, an orbit or cone corresponds to a sublist of {0, 1, …, n }; the entries index the rays that generate the cone.

The projective plane has three fixed points and three fixed curves (under the action of its torus), and projective 3-space has four fixed points, six fixed curves, and four divisors.

i1 : PP2 = projectiveSpace 2;
i2 : orbits(PP2,0)

o2 = {{0, 1}, {0, 2}, {1, 2}}

o2 : List
i3 : orbits(PP2,1)

o3 = {{0}, {1}, {2}}

o3 : List
i4 : PP3 = projectiveSpace 3;
i5 : orbits(PP3,0)

o5 = {{0, 1, 2}, {0, 1, 3}, {0, 2, 3}, {1, 2, 3}}

o5 : List
i6 : orbits(PP3,1)

o6 = {{0, 1}, {0, 2}, {0, 3}, {1, 2}, {1, 3}, {2, 3}}

o6 : List
i7 : orbits(PP3,2)

o7 = {{0}, {1}, {2}, {3}}

o7 : List

Here is a non-simplicial example. Since it is nondegenerate, the fixed points correspond to the maximal cones in the fan. The rays always correspond to the divisors.

i8 : X = normalToricVariety(id_(ZZ^3) | -id_(ZZ^3));
i9 : orbits(X,0)

o9 = {{0, 1, 2, 3}, {0, 1, 4, 5}, {0, 2, 4, 6}, {1, 3, 5, 7}, {2, 3, 6, 7},
     ------------------------------------------------------------------------
     {4, 5, 6, 7}}

o9 : List
i10 : assert(orbits(X,0) === max X)
i11 : orbits(X,1)

o11 = {{0, 1}, {0, 2}, {0, 4}, {1, 3}, {1, 5}, {2, 3}, {2, 6}, {3, 7}, {4,
      -----------------------------------------------------------------------
      5}, {4, 6}, {5, 7}, {6, 7}}

o11 : List
i12 : orbits(X,2)

o12 = {{0}, {1}, {2}, {3}, {4}, {5}, {6}, {7}}

o12 : List
i13 : assert(orbits(X,2) === apply(#rays X, i -> {i}))

The following degenerate example has no fixed points.

i14 : U = normalToricVariety({{4,-1,0},{0,1,0}},{{0,1}});
i15 : assert isDegenerate U
i16 : orbits(U,0)

o16 = {}

o16 : List
i17 : orbits(U,1)

o17 = {{0, 1}}

o17 : List
i18 : orbits(U,2)

o18 = {{0}, {1}}

o18 : List
i19 : dim U

o19 = 3

See also