The Chow ring of M is the ring R := QQ[xF]/(I1 + I2), where I1 = (∑i1∈F xF - ∑i2∈F xF : i1, i2 ∈ M) and I2 = (xFxF’ : F, F’ incomparable), as F runs over all proper nonempty flats of M. This is the same as the Chow ring of the toric variety associated to the Bergman fan of M. This ring is an Artinian standard graded Gorenstein ring, by a result of Adiprasito, Katz, and Huh: cf. https://arxiv.org/abs/1511.02888, Theorem 6.19.
This method returns the defining ideal of the Chow ring, which lives in a polynomial ring with variable indices equal to the flats of M. To work with these subscripts, use "last baseName v" to get the index of a variable v, as shown below:
i1 : M = matroid completeGraph 4 o1 = a matroid of rank 3 on 6 elements o1 : Matroid |
i2 : I = idealChowRing M o2 = ideal (x x , x x , x x , x x , x x , x x , {5} {4} {5} {3} {4} {3} {5} {2} {4} {2} {3} {2} ------------------------------------------------------------------------ x x , x x , x x , x x , x x , x x , x x , {5} {1} {4} {1} {3} {1} {2} {1} {5} {0} {4} {0} {3} {0} ------------------------------------------------------------------------ x x , x x , x x , x x , x x , x x , {2} {0} {1} {0} {4} {0, 5} {3} {0, 5} {2} {0, 5} {1} {0, 5} ------------------------------------------------------------------------ x x , x x , x x , x x , x x , {5} {4, 1} {3} {4, 1} {2} {4, 1} {0} {4, 1} {0, 5} {4, 1} ------------------------------------------------------------------------ x x , x x , x x , x x , x x , x {5} {2, 3} {4} {2, 3} {1} {2, 3} {0} {2, 3} {0, 5} {2, 3} {4, ------------------------------------------------------------------------ x , x x , x x , x x , x x 1} {2, 3} {2} {4, 5, 3} {1} {4, 5, 3} {0} {4, 5, 3} {0, 5} {4, ------------------------------------------------------------------------ , x x , x x , x x , x x 5, 3} {4, 1} {4, 5, 3} {2, 3} {4, 5, 3} {4} {5, 1, 2} {3} {5, 1, ------------------------------------------------------------------------ , x x , x x , x x , x x 2} {0} {5, 1, 2} {0, 5} {5, 1, 2} {4, 1} {5, 1, 2} {2, 3} {5, 1, ------------------------------------------------------------------------ , x x , x x , x x , x x 2} {4, 5, 3} {5, 1, 2} {5} {4, 0, 2} {3} {4, 0, 2} {1} {4, 0, ------------------------------------------------------------------------ , x x , x x , x x , x 2} {0, 5} {4, 0, 2} {4, 1} {4, 0, 2} {2, 3} {4, 0, 2} {4, 5, ------------------------------------------------------------------------ x , x x , x x , x x , 3} {4, 0, 2} {5, 1, 2} {4, 0, 2} {5} {0, 1, 3} {4} {0, 1, 3} ------------------------------------------------------------------------ x x , x x , x x , x x , {2} {0, 1, 3} {0, 5} {0, 1, 3} {4, 1} {0, 1, 3} {2, 3} {0, 1, 3} ------------------------------------------------------------------------ x x , x x , x x , x - {4, 5, 3} {0, 1, 3} {5, 1, 2} {0, 1, 3} {4, 0, 2} {0, 1, 3} {1} ------------------------------------------------------------------------ x - x + x + x - x , x - x - x {0} {0, 5} {4, 1} {5, 1, 2} {4, 0, 2} {2} {0} {0, ------------------------------------------------------------------------ + x + x - x , x - x - x + x 5} {2, 3} {5, 1, 2} {0, 1, 3} {3} {0} {0, 5} {2, 3} ------------------------------------------------------------------------ + x - x , x - x - x + x + x {4, 5, 3} {4, 0, 2} {4} {0} {0, 5} {4, 1} {4, 5, 3} ------------------------------------------------------------------------ - x , x - x + x + x - x - x {0, 1, 3} {5} {0} {4, 5, 3} {5, 1, 2} {4, 0, 2} {0, ------------------------------------------------------------------------ ) 1, 3} o2 : Ideal of QQ[x , x , x , x , x , x , x , x , x , x , x , x , x ] {5} {4} {3} {2} {1} {0} {0, 5} {4, 1} {2, 3} {4, 5, 3} {5, 1, 2} {4, 0, 2} {0, 1, 3} |
i3 : (0..<rank M)/(i -> hilbertFunction(i, I)) o3 = (1, 8, 1) o3 : Sequence |
i4 : apply(gens ring I, v -> last baseName v) o4 = {{5}, {4}, {3}, {2}, {1}, {0}, {0, 5}, {4, 1}, {2, 3}, {4, 5, 3}, {5, 1, ------------------------------------------------------------------------ 2}, {4, 0, 2}, {0, 1, 3}} o4 : List |