This is an example of application of the method projectiveDegrees, due to results shown in Computing characteristic classes of projective schemes, by P. Aluffi. See also the corresponding methods in the packages CSM-A, by P. Aluffi, and CharacteristicClasses, by M. Helmer and C. Jost.
In the example below, we compute the push-forward to the Chow ring of $\mathbb{P}^4$ of the Chern-Schwartz-MacPherson class of the cone over the twisted cubic curve, using both a probabilistic and a non-probabilistic approach.
i1 : GF(5^7)[x_0..x_4] o1 = GF 78125[x ..x ] 0 4 o1 : PolynomialRing |
i2 : C = minors(2,matrix{{x_0,x_1,x_2},{x_1,x_2,x_3}}) 2 2 o2 = ideal (- x + x x , - x x + x x , - x + x x ) 1 0 2 1 2 0 3 2 1 3 o2 : Ideal of GF 78125[x ..x ] 0 4 |
i3 : time ChernSchwartzMacPherson C -- used 1.51463 seconds 4 3 2 o3 = 3H + 5H + 3H ZZ[H] o3 : ----- 5 H |
i4 : time ChernSchwartzMacPherson(C,MathMode=>true) MathMode: output certified! -- used 1.99 seconds 4 3 2 o4 = 3H + 5H + 3H ZZ[H] o4 : ----- 5 H |
i5 : oo == ooo o5 = true |
In the case when the input ideal I defines a smooth projective variety $X$, the push-forward of $c_{SM}(X)$ can be computed much more efficiently using SegreClass. Indeed, in this case, $c_{SM}(X)$ coincides with the (total) Chern class of the tangent bundle of $X$ and can be obtained as follows (in general the method below gives the push-forward of the so-called Chern-Fulton class).
i6 : ChernClass = method(Options=>{MathMode=>false}); |
i7 : ChernClass (Ideal) := o -> (I) -> ( s := SegreClass(I,MathMode=>o.MathMode); s*(1+first gens ring s)^(numgens ring I)); |
i8 : -- example: Chern class of G(1,4) G = Grassmannian(1,4,CoefficientRing=>ZZ/190181) o8 = ideal (p p - p p + p p , p p - p p + p p , p p - p p + p p , p p - p p + 2,3 1,4 1,3 2,4 1,2 3,4 2,3 0,4 0,3 2,4 0,2 3,4 1,3 0,4 0,3 1,4 0,1 3,4 1,2 0,4 0,2 1,4 ---------------------------------------------------------------------------------------------------------------------------- p p , p p - p p + p p ) 0,1 2,4 1,2 0,3 0,2 1,3 0,1 2,3 ZZ o8 : Ideal of ------[p ..p , p , p , p , p , p , p , p , p ] 190181 0,1 0,2 1,2 0,3 1,3 2,3 0,4 1,4 2,4 3,4 |
i9 : time ChernClass G -- used 0.193069 seconds 9 8 7 6 5 4 3 o9 = 10H + 30H + 60H + 75H + 57H + 25H + 5H ZZ[H] o9 : ----- 10 H |
i10 : time ChernClass(G,MathMode=>true) MathMode: output certified! -- used 0.0605401 seconds 9 8 7 6 5 4 3 o10 = 10H + 30H + 60H + 75H + 57H + 25H + 5H ZZ[H] o10 : ----- 10 H |
The object ChernSchwartzMacPherson is a method function with options.