MAGMA Computational Algebra System

In this example, we consider a surface X from a special family of rational surfaces of degree 10 in Prj⁴. This family is described by Decker, Ein and Schreyer in Section 2.1 of [DES93]. They have sectional genus 9 and are isomorphic to the plane blown up in 18 points in special position which give 18 exceptional curves in X. The embedding into Prj⁴ is such that 4 of these exceptional curves are of degree 3, 7 are of degree 2 and 7 are of degree 1.

The adjunction map on X (the map corresponding to the divisor K_X + H where K_X is a canonical divisor and H a hyperplane section or equivalently to the sheaf ( K)_X(1) where ( K)_X is the canonical sheaf) maps X to a smooth surface X₁ of degree 13 in Prj⁸ blowing down the 7 degree 1 exceptional curves to points and reducing the degrees of the others by 1. The adjunction map on X₁ blows down the 7 exceptional curves originally of degree 2 to points and maps X₂ to an anticanonically embedded degree 5 Del Pezzo surface in Prj⁵.

We take a randomly generated surface from this family over a small finite field (F₁₇) and illustrate this process by explicitly computing the adjunction maps and images X₁ and X₂. We show that the intersection pairings of the canonical divisor and hyperplane sections on X, X₁, X₂ are as they should be and that X₂ really is an anticanonically embedded Del Pezzo. We also expand the composition of the 2 divisor maps and show that the result is indeed birational from X onto X₂.

These surfaces X are defined by one degree 4 and 10 degree 5 polynomials in Prj⁴. The embedding is quite a complex one and it is hard to construct one with defining polynomials which are at all sparse. This makes it a fairly challenging for explicit computation but means that an example takes up a lot of page space! An example with relatively small coefficients over Q can also be processed, though the total running time is a few minutes. Also the resulting X₂ tends to have very large coefficients. Here we get no coefficient blow-up and X₂ is a much simpler looking surface than X.

> P<[x]> := ProjectiveSpace(GF(17),4);
> X := Scheme(P, [
>    10*x[1]^4 + 13*x[1]^3*x[2] + 8*x[1]*x[2]^3 + 4*x[2]^4 + 6*x[1]^3*x[3] + 
>        15*x[1]^2*x[2]*x[3] + 14*x[2]^3*x[3] + x[1]^2*x[3]^2 + 
>        13*x[1]*x[2]*x[3]^2 + 3*x[2]^2*x[3]^2 + 9*x[1]*x[3]^3 + 2*x[2]*x[3]^3 + 
>        10*x[3]^4 + 15*x[1]^3*x[4] + 4*x[1]^2*x[2]*x[4] + 3*x[1]*x[2]^2*x[4] + 
>        7*x[2]^3*x[4] + 9*x[1]^2*x[3]*x[4] + 3*x[1]*x[2]*x[3]*x[4] + 
>        9*x[2]^2*x[3]*x[4] + 11*x[1]*x[3]^2*x[4] + 6*x[2]*x[3]^2*x[4] + 
>        15*x[3]^3*x[4] + x[1]^2*x[4]^2 + 4*x[1]*x[2]*x[4]^2 + 2*x[2]^2*x[4]^2 + 
>        12*x[1]*x[3]*x[4]^2 + 8*x[2]*x[3]*x[4]^2 + 9*x[3]^2*x[4]^2 + 
>        10*x[1]*x[4]^3 + 5*x[2]*x[4]^3 + 14*x[3]*x[4]^3 + 4*x[1]^3*x[5] + 
>        16*x[1]^2*x[2]*x[5] + 15*x[2]^3*x[5] + 13*x[1]^2*x[3]*x[5] + 
>        13*x[1]*x[2]*x[3]*x[5] + 10*x[2]^2*x[3]*x[5] + 15*x[1]*x[3]^2*x[5] + 
>        7*x[2]*x[3]^2*x[5] + 14*x[3]^3*x[5] + 11*x[1]^2*x[4]*x[5] + 
>        10*x[1]*x[2]*x[4]*x[5] + 4*x[2]^2*x[4]*x[5] + x[1]*x[3]*x[4]*x[5] + 
>        12*x[2]*x[3]*x[4]*x[5] + 8*x[3]^2*x[4]*x[5] + 5*x[1]*x[4]^2*x[5] + 
>        5*x[2]*x[4]^2*x[5] + 11*x[3]*x[4]^2*x[5] + 10*x[4]^3*x[5] + 
>        12*x[1]^2*x[5]^2 + 8*x[1]*x[2]*x[5]^2 + 16*x[2]^2*x[5]^2 + 
>        12*x[1]*x[3]*x[5]^2 + x[2]*x[3]*x[5]^2 + 14*x[3]^2*x[5]^2 + 
>        8*x[1]*x[4]*x[5]^2 + x[2]*x[4]*x[5]^2 + 3*x[3]*x[4]*x[5]^2 + 
>        5*x[4]^2*x[5]^2 + 11*x[1]*x[5]^3 + 13*x[2]*x[5]^3 + 5*x[3]*x[5]^3 + 
>        9*x[4]*x[5]^3 + 8*x[5]^4,
>    9*x[1]^4*x[4] + 14*x[1]^3*x[2]*x[4] + 5*x[1]^2*x[2]^2*x[4] + 
>        2*x[1]*x[2]^3*x[4] + 2*x[1]^3*x[3]*x[4] + 7*x[1]^2*x[2]*x[3]*x[4] + 
>        5*x[1]*x[2]^2*x[3]*x[4] + 7*x[2]^3*x[3]*x[4] + 9*x[1]^2*x[3]^2*x[4] + 
>        12*x[1]*x[2]*x[3]^2*x[4] + 2*x[2]^2*x[3]^2*x[4] + 9*x[1]*x[3]^3*x[4] + 
>        2*x[2]*x[3]^3*x[4] + x[3]^4*x[4] + 3*x[1]^3*x[4]^2 + 
>        5*x[1]^2*x[2]*x[4]^2 + 7*x[1]*x[2]^2*x[4]^2 + 13*x[2]^3*x[4]^2 + 
>        11*x[1]^2*x[3]*x[4]^2 + 4*x[1]*x[2]*x[3]*x[4]^2 + 11*x[2]^2*x[3]*x[4]^2 
>        + 14*x[1]*x[3]^2*x[4]^2 + 16*x[2]*x[3]^2*x[4]^2 + 15*x[1]^2*x[4]^3 + 
>        11*x[1]*x[2]*x[4]^3 + 5*x[2]^2*x[4]^3 + 6*x[1]*x[3]*x[4]^3 + 
>        9*x[2]*x[3]*x[4]^3 + 16*x[3]^2*x[4]^3 + 9*x[2]*x[4]^4 + 15*x[3]*x[4]^4 +
>        14*x[4]^5 + 2*x[1]^3*x[2]*x[5] + 6*x[1]^2*x[2]^2*x[5] + 
>        3*x[1]*x[2]^3*x[5] + 16*x[2]^4*x[5] + 15*x[1]^3*x[3]*x[5] + 
>        6*x[1]*x[2]^2*x[3]*x[5] + 10*x[2]^3*x[3]*x[5] + 14*x[1]^2*x[3]^2*x[5] + 
>        13*x[1]*x[2]*x[3]^2*x[5] + 4*x[2]^2*x[3]^2*x[5] + 16*x[1]*x[3]^3*x[5] + 
>        13*x[3]^4*x[5] + 14*x[1]^3*x[4]*x[5] + 9*x[1]^2*x[2]*x[4]*x[5] + 
>        16*x[1]*x[2]^2*x[4]*x[5] + 14*x[2]^3*x[4]*x[5] + 
>        6*x[1]*x[2]*x[3]*x[4]*x[5] + 6*x[2]^2*x[3]*x[4]*x[5] + 
>        3*x[1]*x[3]^2*x[4]*x[5] + 7*x[2]*x[3]^2*x[4]*x[5] + 7*x[3]^3*x[4]*x[5] +
>        2*x[1]^2*x[4]^2*x[5] + 15*x[1]*x[2]*x[4]^2*x[5] + 
>        9*x[1]*x[3]*x[4]^2*x[5] + 14*x[3]^2*x[4]^2*x[5] + 14*x[1]*x[4]^3*x[5] + 
>        6*x[2]*x[4]^3*x[5] + 12*x[3]*x[4]^3*x[5] + 3*x[4]^4*x[5] + 
>        9*x[1]^3*x[5]^2 + 12*x[1]^2*x[2]*x[5]^2 + 16*x[1]*x[2]^2*x[5]^2 + 
>        x[2]^3*x[5]^2 + 7*x[1]^2*x[3]*x[5]^2 + 5*x[1]*x[2]*x[3]*x[5]^2 + 
>        8*x[2]^2*x[3]*x[5]^2 + 2*x[1]*x[3]^2*x[5]^2 + 4*x[2]*x[3]^2*x[5]^2 + 
>        13*x[3]^3*x[5]^2 + 7*x[1]^2*x[4]*x[5]^2 + 6*x[2]^2*x[4]*x[5]^2 + 
>        16*x[1]*x[3]*x[4]*x[5]^2 + 15*x[2]*x[3]*x[4]*x[5]^2 + 
>        7*x[3]^2*x[4]*x[5]^2 + 6*x[1]*x[4]^2*x[5]^2 + 3*x[2]*x[4]^2*x[5]^2 + 
>        16*x[3]*x[4]^2*x[5]^2 + 15*x[4]^3*x[5]^2 + x[1]^2*x[5]^3 + 
>        13*x[1]*x[2]*x[5]^3 + 6*x[2]^2*x[5]^3 + 8*x[1]*x[3]*x[5]^3 + 
>        x[2]*x[3]*x[5]^3 + 9*x[3]^2*x[5]^3 + 3*x[1]*x[4]*x[5]^3 + 
>        14*x[2]*x[4]*x[5]^3 + 8*x[3]*x[4]*x[5]^3 + 14*x[4]^2*x[5]^3 + 
>        16*x[1]*x[5]^4 + 2*x[2]*x[5]^4 + 7*x[3]*x[5]^4 + 7*x[4]*x[5]^4 + 
>        11*x[5]^5,
>    13*x[1]^4*x[4] + 8*x[1]^3*x[2]*x[4] + 14*x[1]^2*x[2]^2*x[4] + 
>        3*x[1]*x[2]^3*x[4] + 11*x[2]^4*x[4] + 7*x[1]^3*x[3]*x[4] + 
>        3*x[1]^2*x[2]*x[3]*x[4] + 12*x[2]^3*x[3]*x[4] + 3*x[1]^2*x[3]^2*x[4] + 
>        13*x[1]*x[2]*x[3]^2*x[4] + 3*x[2]^2*x[3]^2*x[4] + 7*x[1]*x[3]^3*x[4] + 
>        2*x[2]*x[3]^3*x[4] + 7*x[3]^4*x[4] + 13*x[1]^3*x[4]^2 + 
>        6*x[1]^2*x[2]*x[4]^2 + 6*x[1]*x[2]^2*x[4]^2 + 6*x[2]^3*x[4]^2 + 
>        2*x[1]^2*x[3]*x[4]^2 + 15*x[1]*x[2]*x[3]*x[4]^2 + 14*x[2]^2*x[3]*x[4]^2 
>        + 3*x[1]*x[3]^2*x[4]^2 + 16*x[2]*x[3]^2*x[4]^2 + 3*x[3]^3*x[4]^2 + 
>        6*x[1]^2*x[4]^3 + 10*x[2]^2*x[4]^3 + 7*x[2]*x[3]*x[4]^3 + 13*x[1]*x[4]^4
>        + 5*x[2]*x[4]^4 + 15*x[3]*x[4]^4 + 13*x[4]^5 + 2*x[1]^4*x[5] + 
>        6*x[1]^3*x[2]*x[5] + 12*x[1]^2*x[2]^2*x[5] + 12*x[1]*x[2]^3*x[5] + 
>        2*x[2]^4*x[5] + 5*x[1]^3*x[3]*x[5] + 12*x[1]^2*x[2]*x[3]*x[5] + 
>        7*x[1]*x[2]^2*x[3]*x[5] + 11*x[2]^3*x[3]*x[5] + 2*x[1]^2*x[3]^2*x[5] + 
>        3*x[1]*x[2]*x[3]^2*x[5] + 7*x[2]^2*x[3]^2*x[5] + 16*x[1]*x[3]^3*x[5] + 
>        3*x[2]*x[3]^3*x[5] + 13*x[3]^4*x[5] + 2*x[1]^2*x[2]*x[4]*x[5] + 
>        12*x[1]*x[2]^2*x[4]*x[5] + 2*x[2]^3*x[4]*x[5] + 10*x[1]^2*x[3]*x[4]*x[5]
>        + 9*x[1]*x[2]*x[3]*x[4]*x[5] + 6*x[2]^2*x[3]*x[4]*x[5] + 
>        x[1]*x[3]^2*x[4]*x[5] + 6*x[2]*x[3]^2*x[4]*x[5] + 15*x[3]^3*x[4]*x[5] + 
>        2*x[1]^2*x[4]^2*x[5] + 14*x[1]*x[2]*x[4]^2*x[5] + 
>        13*x[1]*x[3]*x[4]^2*x[5] + 13*x[2]*x[3]*x[4]^2*x[5] + 
>        2*x[3]^2*x[4]^2*x[5] + 12*x[1]*x[4]^3*x[5] + 8*x[2]*x[4]^3*x[5] + 
>        8*x[3]*x[4]^3*x[5] + x[4]^4*x[5] + 3*x[1]^3*x[5]^2 + 
>        7*x[1]^2*x[2]*x[5]^2 + 4*x[1]^2*x[3]*x[5]^2 + 3*x[1]*x[2]*x[3]*x[5]^2 + 
>        9*x[2]^2*x[3]*x[5]^2 + 14*x[1]*x[3]^2*x[5]^2 + 13*x[2]*x[3]^2*x[5]^2 + 
>        15*x[3]^3*x[5]^2 + x[1]^2*x[4]*x[5]^2 + 14*x[1]*x[2]*x[4]*x[5]^2 + 
>        5*x[2]^2*x[4]*x[5]^2 + 10*x[1]*x[3]*x[4]*x[5]^2 + 
>        5*x[2]*x[3]*x[4]*x[5]^2 + 7*x[3]^2*x[4]*x[5]^2 + 13*x[1]*x[4]^2*x[5]^2 +
>        2*x[2]*x[4]^2*x[5]^2 + 9*x[3]*x[4]^2*x[5]^2 + 3*x[4]^3*x[5]^2 + 
>        14*x[1]*x[2]*x[5]^3 + 12*x[2]^2*x[5]^3 + 6*x[1]*x[3]*x[5]^3 + 
>        16*x[2]*x[3]*x[5]^3 + 8*x[3]^2*x[5]^3 + 3*x[1]*x[4]*x[5]^3 + 
>        4*x[2]*x[4]*x[5]^3 + 11*x[3]*x[4]*x[5]^3 + 15*x[4]^2*x[5]^3 + 
>        14*x[1]*x[5]^4 + 13*x[2]*x[5]^4 + 4*x[3]*x[5]^4 + 4*x[4]*x[5]^4 + 
>        13*x[5]^5,
>    15*x[1]^3*x[2]*x[3] + 11*x[1]^2*x[2]^2*x[3] + 14*x[1]*x[2]^3*x[3] + 
>        x[2]^4*x[3] + 2*x[1]^3*x[3]^2 + 11*x[1]*x[2]^2*x[3]^2 + 7*x[2]^3*x[3]^2 
>        + 3*x[1]^2*x[3]^3 + 4*x[1]*x[2]*x[3]^3 + 13*x[2]^2*x[3]^3 + x[1]*x[3]^4 
>        + 4*x[3]^5 + 2*x[1]^4*x[4] + 11*x[1]^3*x[2]*x[4] + 13*x[1]^2*x[2]^2*x[4]
>        + 4*x[1]*x[2]^3*x[4] + 16*x[2]^4*x[4] + 5*x[1]^3*x[3]*x[4] + 
>        4*x[1]^2*x[2]*x[3]*x[4] + 10*x[1]*x[2]^2*x[3]*x[4] + 8*x[2]^3*x[3]*x[4] 
>        + 5*x[1]^2*x[3]^2*x[4] + 14*x[1]*x[2]*x[3]^2*x[4] + 2*x[2]^2*x[3]^2*x[4]
>        + 15*x[1]*x[3]^3*x[4] + 13*x[3]^4*x[4] + 9*x[1]^3*x[4]^2 + 
>        3*x[1]^2*x[2]*x[4]^2 + 10*x[1]*x[2]^2*x[4]^2 + 12*x[2]^3*x[4]^2 + 
>        8*x[1]^2*x[3]*x[4]^2 + 14*x[1]*x[2]*x[3]*x[4]^2 + 3*x[2]^2*x[3]*x[4]^2 +
>        2*x[1]*x[3]^2*x[4]^2 + 5*x[2]*x[3]^2*x[4]^2 + 10*x[3]^3*x[4]^2 + 
>        5*x[1]^2*x[4]^3 + x[1]*x[2]*x[4]^3 + 8*x[2]^2*x[4]^3 + 
>        7*x[1]*x[3]*x[4]^3 + 10*x[2]*x[3]*x[4]^3 + 13*x[3]^2*x[4]^3 + 
>        10*x[1]*x[4]^4 + 7*x[2]*x[4]^4 + 16*x[3]*x[4]^4 + 16*x[4]^5 + 
>        8*x[1]^3*x[3]*x[5] + 5*x[1]^2*x[2]*x[3]*x[5] + x[1]*x[2]^2*x[3]*x[5] + 
>        16*x[2]^3*x[3]*x[5] + 10*x[1]^2*x[3]^2*x[5] + 12*x[1]*x[2]*x[3]^2*x[5] +
>        9*x[2]^2*x[3]^2*x[5] + 15*x[1]*x[3]^3*x[5] + 13*x[2]*x[3]^3*x[5] + 
>        4*x[3]^4*x[5] + 14*x[1]^3*x[4]*x[5] + x[1]^2*x[2]*x[4]*x[5] + 
>        10*x[1]*x[2]^2*x[4]*x[5] + 11*x[2]^3*x[4]*x[5] + 5*x[1]^2*x[3]*x[4]*x[5]
>        + 12*x[1]*x[2]*x[3]*x[4]*x[5] + 7*x[2]^2*x[3]*x[4]*x[5] + 
>        5*x[1]*x[3]^2*x[4]*x[5] + 3*x[3]^3*x[4]*x[5] + 2*x[1]^2*x[4]^2*x[5] + 
>        5*x[2]^2*x[4]^2*x[5] + 2*x[2]*x[3]*x[4]^2*x[5] + 8*x[3]^2*x[4]^2*x[5] + 
>        x[1]*x[4]^3*x[5] + 5*x[2]*x[4]^3*x[5] + 3*x[3]*x[4]^3*x[5] + 
>        14*x[4]^4*x[5] + 16*x[1]^2*x[3]*x[5]^2 + 4*x[1]*x[2]*x[3]*x[5]^2 + 
>        11*x[2]^2*x[3]*x[5]^2 + 9*x[1]*x[3]^2*x[5]^2 + 16*x[2]*x[3]^2*x[5]^2 + 
>        8*x[3]^3*x[5]^2 + 8*x[1]^2*x[4]*x[5]^2 + 11*x[1]*x[2]*x[4]*x[5]^2 + 
>        3*x[2]^2*x[4]*x[5]^2 + 6*x[1]*x[3]*x[4]*x[5]^2 + 9*x[2]*x[3]*x[4]*x[5]^2
>        + 5*x[3]^2*x[4]*x[5]^2 + 15*x[1]*x[4]^2*x[5]^2 + 2*x[2]*x[4]^2*x[5]^2 + 
>        8*x[3]*x[4]^2*x[5]^2 + 14*x[4]^3*x[5]^2 + x[1]*x[3]*x[5]^3 + 
>        15*x[2]*x[3]*x[5]^3 + 10*x[3]^2*x[5]^3 + 11*x[1]*x[4]*x[5]^3 + 
>        8*x[2]*x[4]*x[5]^3 + 15*x[3]*x[4]*x[5]^3 + 15*x[4]^2*x[5]^3 + 
>        6*x[3]*x[5]^4 + 3*x[4]*x[5]^4,
>    9*x[1]^4*x[3] + 14*x[1]^3*x[2]*x[3] + 5*x[1]^2*x[2]^2*x[3] + 
>        2*x[1]*x[2]^3*x[3] + 2*x[1]^3*x[3]^2 + 7*x[1]^2*x[2]*x[3]^2 + 
>        5*x[1]*x[2]^2*x[3]^2 + 7*x[2]^3*x[3]^2 + 9*x[1]^2*x[3]^3 + 
>        12*x[1]*x[2]*x[3]^3 + 2*x[2]^2*x[3]^3 + 9*x[1]*x[3]^4 + 2*x[2]*x[3]^4 + 
>        x[3]^5 + 3*x[1]^3*x[3]*x[4] + 5*x[1]^2*x[2]*x[3]*x[4] + 
>        7*x[1]*x[2]^2*x[3]*x[4] + 13*x[2]^3*x[3]*x[4] + 11*x[1]^2*x[3]^2*x[4] + 
>        4*x[1]*x[2]*x[3]^2*x[4] + 11*x[2]^2*x[3]^2*x[4] + 14*x[1]*x[3]^3*x[4] + 
>        16*x[2]*x[3]^3*x[4] + 15*x[1]^2*x[3]*x[4]^2 + 11*x[1]*x[2]*x[3]*x[4]^2 +
>        5*x[2]^2*x[3]*x[4]^2 + 6*x[1]*x[3]^2*x[4]^2 + 9*x[2]*x[3]^2*x[4]^2 + 
>        16*x[3]^3*x[4]^2 + 9*x[2]*x[3]*x[4]^3 + 15*x[3]^2*x[4]^3 + 
>        14*x[3]*x[4]^4 + 2*x[1]^4*x[5] + 11*x[1]^3*x[2]*x[5] + 
>        13*x[1]^2*x[2]^2*x[5] + 4*x[1]*x[2]^3*x[5] + 16*x[2]^4*x[5] + 
>        2*x[1]^3*x[3]*x[5] + 13*x[1]^2*x[2]*x[3]*x[5] + 9*x[1]*x[2]^2*x[3]*x[5] 
>        + 5*x[2]^3*x[3]*x[5] + 5*x[1]^2*x[3]^2*x[5] + 3*x[1]*x[2]*x[3]^2*x[5] + 
>        8*x[2]^2*x[3]^2*x[5] + x[1]*x[3]^3*x[5] + 7*x[2]*x[3]^3*x[5] + 
>        3*x[3]^4*x[5] + 9*x[1]^3*x[4]*x[5] + 3*x[1]^2*x[2]*x[4]*x[5] + 
>        10*x[1]*x[2]^2*x[4]*x[5] + 12*x[2]^3*x[4]*x[5] + 
>        10*x[1]^2*x[3]*x[4]*x[5] + 12*x[1]*x[2]*x[3]*x[4]*x[5] + 
>        3*x[2]^2*x[3]*x[4]*x[5] + 11*x[1]*x[3]^2*x[4]*x[5] + 
>        5*x[2]*x[3]^2*x[4]*x[5] + 7*x[3]^3*x[4]*x[5] + 5*x[1]^2*x[4]^2*x[5] + 
>        x[1]*x[2]*x[4]^2*x[5] + 8*x[2]^2*x[4]^2*x[5] + 4*x[1]*x[3]*x[4]^2*x[5] +
>        16*x[2]*x[3]*x[4]^2*x[5] + 8*x[3]^2*x[4]^2*x[5] + 10*x[1]*x[4]^3*x[5] + 
>        7*x[2]*x[4]^3*x[5] + 2*x[3]*x[4]^3*x[5] + 16*x[4]^4*x[5] + 
>        14*x[1]^3*x[5]^2 + x[1]^2*x[2]*x[5]^2 + 10*x[1]*x[2]^2*x[5]^2 + 
>        11*x[2]^3*x[5]^2 + 12*x[1]^2*x[3]*x[5]^2 + 12*x[1]*x[2]*x[3]*x[5]^2 + 
>        13*x[2]^2*x[3]*x[5]^2 + 4*x[1]*x[3]^2*x[5]^2 + 15*x[2]*x[3]^2*x[5]^2 + 
>        10*x[3]^3*x[5]^2 + 2*x[1]^2*x[4]*x[5]^2 + 5*x[2]^2*x[4]*x[5]^2 + 
>        6*x[1]*x[3]*x[4]*x[5]^2 + 5*x[2]*x[3]*x[4]*x[5]^2 + 7*x[3]^2*x[4]*x[5]^2
>        + x[1]*x[4]^2*x[5]^2 + 5*x[2]*x[4]^2*x[5]^2 + x[3]*x[4]^2*x[5]^2 + 
>        14*x[4]^3*x[5]^2 + 8*x[1]^2*x[5]^3 + 11*x[1]*x[2]*x[5]^3 + 
>        3*x[2]^2*x[5]^3 + 9*x[1]*x[3]*x[5]^3 + 6*x[2]*x[3]*x[5]^3 + 
>        13*x[3]^2*x[5]^3 + 15*x[1]*x[4]*x[5]^3 + 2*x[2]*x[4]*x[5]^3 + 
>        5*x[3]*x[4]*x[5]^3 + 14*x[4]^2*x[5]^3 + 11*x[1]*x[5]^4 + 8*x[2]*x[5]^4 +
>        5*x[3]*x[5]^4 + 15*x[4]*x[5]^4 + 3*x[5]^5,
>    13*x[1]^4*x[3] + 8*x[1]^3*x[2]*x[3] + 14*x[1]^2*x[2]^2*x[3] + 
>        3*x[1]*x[2]^3*x[3] + 11*x[2]^4*x[3] + 7*x[1]^3*x[3]^2 + 
>        3*x[1]^2*x[2]*x[3]^2 + 12*x[2]^3*x[3]^2 + 3*x[1]^2*x[3]^3 + 
>        13*x[1]*x[2]*x[3]^3 + 3*x[2]^2*x[3]^3 + 7*x[1]*x[3]^4 + 2*x[2]*x[3]^4 + 
>        7*x[3]^5 + 13*x[1]^3*x[3]*x[4] + 6*x[1]^2*x[2]*x[3]*x[4] + 
>        6*x[1]*x[2]^2*x[3]*x[4] + 6*x[2]^3*x[3]*x[4] + 2*x[1]^2*x[3]^2*x[4] +
>        15*x[1]*x[2]*x[3]^2*x[4] + 14*x[2]^2*x[3]^2*x[4] + 3*x[1]*x[3]^3*x[4] +  
>        16*x[2]*x[3]^3*x[4] + 3*x[3]^4*x[4] + 6*x[1]^2*x[3]*x[4]^2 + 
>        10*x[2]^2*x[3]*x[4]^2 + 7*x[2]*x[3]^2*x[4]^2 + 13*x[1]*x[3]*x[4]^3 + 
>        5*x[2]*x[3]*x[4]^3 + 15*x[3]^2*x[4]^3 + 13*x[3]*x[4]^4 + 15*x[1]^4*x[5] 
>        + 15*x[1]^3*x[2]*x[5] + 2*x[1]^2*x[2]^2*x[5] + 16*x[1]*x[2]^3*x[5] + 
>        16*x[2]^4*x[5] + 14*x[1]^3*x[3]*x[5] + 4*x[1]^2*x[2]*x[3]*x[5] + 
>        10*x[1]*x[2]^2*x[3]*x[5] + 4*x[2]^3*x[3]*x[5] + 8*x[1]^2*x[3]^2*x[5] + 
>        5*x[1]*x[2]*x[3]^2*x[5] + 11*x[2]^2*x[3]^2*x[5] + 12*x[2]*x[3]^3*x[5] + 
>        2*x[3]^4*x[5] + 15*x[1]^2*x[2]*x[4]*x[5] + 6*x[2]^3*x[4]*x[5] + 
>        9*x[1]^2*x[3]*x[4]*x[5] + 9*x[1]*x[2]*x[3]*x[4]*x[5] + 
>        15*x[2]^2*x[3]*x[4]*x[5] + 14*x[1]*x[3]^2*x[4]*x[5] + 
>        13*x[2]*x[3]^2*x[4]*x[5] + 6*x[3]^3*x[4]*x[5] + 4*x[1]^2*x[4]^2*x[5] + 
>        7*x[1]*x[2]*x[4]^2*x[5] + 3*x[2]^2*x[4]^2*x[5] + 8*x[1]*x[3]*x[4]^2*x[5]
>        + 8*x[2]*x[3]*x[4]^2*x[5] + 3*x[3]^2*x[4]^2*x[5] + 15*x[1]*x[4]^3*x[5] +
>        3*x[2]*x[4]^3*x[5] + 8*x[3]*x[4]^3*x[5] + 2*x[4]^4*x[5] + 
>        2*x[1]^3*x[5]^2 + 6*x[1]^2*x[2]*x[5]^2 + x[1]*x[2]^2*x[5]^2 + 
>        7*x[2]^3*x[5]^2 + 3*x[1]^2*x[3]*x[5]^2 + 16*x[1]*x[2]*x[3]*x[5]^2 + 
>        10*x[2]^2*x[3]*x[5]^2 + 10*x[1]*x[3]^2*x[5]^2 + 13*x[2]*x[3]^2*x[5]^2 + 
>        2*x[3]^3*x[5]^2 + 4*x[1]^2*x[4]*x[5]^2 + x[1]*x[2]*x[4]*x[5]^2 + 
>        9*x[2]^2*x[4]*x[5]^2 + 16*x[1]*x[3]*x[4]*x[5]^2 + 
>        8*x[2]*x[3]*x[4]*x[5]^2 + 11*x[1]*x[4]^2*x[5]^2 + 11*x[2]*x[4]^2*x[5]^2 
>        + 4*x[3]*x[4]^2*x[5]^2 + 10*x[4]^3*x[5]^2 + 10*x[1]^2*x[5]^3 + 
>        14*x[2]^2*x[5]^3 + 16*x[1]*x[3]*x[5]^3 + 13*x[2]*x[3]*x[5]^3 + 
>        15*x[3]^2*x[5]^3 + 16*x[1]*x[4]*x[5]^3 + 3*x[2]*x[4]*x[5]^3 + 
>        4*x[3]*x[4]*x[5]^3 + 2*x[4]^2*x[5]^3 + x[1]*x[5]^4 + 7*x[2]*x[5]^4 + 
>        7*x[4]*x[5]^4 + 2*x[5]^5,
>    15*x[1]^4*x[3] + 11*x[1]^3*x[2]*x[3] + 5*x[1]^2*x[2]^2*x[3] + 
>        5*x[1]*x[2]^3*x[3] + 15*x[2]^4*x[3] + 12*x[1]^3*x[3]^2 + 
>        5*x[1]^2*x[2]*x[3]^2 + 10*x[1]*x[2]^2*x[3]^2 + 6*x[2]^3*x[3]^2 + 
>        15*x[1]^2*x[3]^3 + 14*x[1]*x[2]*x[3]^3 + 10*x[2]^2*x[3]^3 + x[1]*x[3]^4 
>        + 14*x[2]*x[3]^4 + 4*x[3]^5 + 15*x[1]^4*x[4] + 15*x[1]^3*x[2]*x[4] + 
>        2*x[1]^2*x[2]^2*x[4] + 16*x[1]*x[2]^3*x[4] + 16*x[2]^4*x[4] + 
>        14*x[1]^3*x[3]*x[4] + 2*x[1]^2*x[2]*x[3]*x[4] + 15*x[1]*x[2]^2*x[3]*x[4]
>        + 2*x[2]^3*x[3]*x[4] + 15*x[1]^2*x[3]^2*x[4] + 13*x[1]*x[2]*x[3]^2*x[4] 
>        + 5*x[2]^2*x[3]^2*x[4] + 16*x[1]*x[3]^3*x[4] + 6*x[2]*x[3]^3*x[4] + 
>        4*x[3]^4*x[4] + 15*x[1]^2*x[2]*x[4]^2 + 6*x[2]^3*x[4]^2 + 
>        7*x[1]^2*x[3]*x[4]^2 + 12*x[1]*x[2]*x[3]*x[4]^2 + 15*x[2]^2*x[3]*x[4]^2 
>        + x[1]*x[3]^2*x[4]^2 + 4*x[3]^3*x[4]^2 + 4*x[1]^2*x[4]^3 + 
>        7*x[1]*x[2]*x[4]^3 + 3*x[2]^2*x[4]^3 + 13*x[1]*x[3]*x[4]^3 + 
>        12*x[3]^2*x[4]^3 + 15*x[1]*x[4]^4 + 3*x[2]*x[4]^4 + 7*x[3]*x[4]^4 + 
>        2*x[4]^5 + 14*x[1]^3*x[3]*x[5] + 10*x[1]^2*x[2]*x[3]*x[5] + 
>        13*x[1]^2*x[3]^2*x[5] + 14*x[1]*x[2]*x[3]^2*x[5] + 8*x[2]^2*x[3]^2*x[5] 
>        + 3*x[1]*x[3]^3*x[5] + 4*x[2]*x[3]^3*x[5] + 2*x[3]^4*x[5] + 
>        2*x[1]^3*x[4]*x[5] + 6*x[1]^2*x[2]*x[4]*x[5] + x[1]*x[2]^2*x[4]*x[5] + 
>        7*x[2]^3*x[4]*x[5] + 2*x[1]^2*x[3]*x[4]*x[5] + 
>        2*x[1]*x[2]*x[3]*x[4]*x[5] + 5*x[2]^2*x[3]*x[4]*x[5] + 
>        8*x[2]*x[3]^2*x[4]*x[5] + 12*x[3]^3*x[4]*x[5] + 4*x[1]^2*x[4]^2*x[5] + 
>        x[1]*x[2]*x[4]^2*x[5] + 9*x[2]^2*x[4]^2*x[5] + 3*x[1]*x[3]*x[4]^2*x[5] +
>        6*x[2]*x[3]*x[4]^2*x[5] + 8*x[3]^2*x[4]^2*x[5] + 11*x[1]*x[4]^3*x[5] + 
>        11*x[2]*x[4]^3*x[5] + x[3]*x[4]^3*x[5] + 10*x[4]^4*x[5] + 
>        3*x[1]*x[2]*x[3]*x[5]^2 + 5*x[2]^2*x[3]*x[5]^2 + 11*x[1]*x[3]^2*x[5]^2 +
>        x[2]*x[3]^2*x[5]^2 + 9*x[3]^3*x[5]^2 + 10*x[1]^2*x[4]*x[5]^2 + 
>        14*x[2]^2*x[4]*x[5]^2 + 13*x[1]*x[3]*x[4]*x[5]^2 + 
>        9*x[2]*x[3]*x[4]*x[5]^2 + 4*x[3]^2*x[4]*x[5]^2 + 16*x[1]*x[4]^2*x[5]^2 +
>        3*x[2]*x[4]^2*x[5]^2 + 6*x[3]*x[4]^2*x[5]^2 + 2*x[4]^3*x[5]^2 + 
>        3*x[1]*x[3]*x[5]^3 + 4*x[2]*x[3]*x[5]^3 + 13*x[3]^2*x[5]^3 + 
>        x[1]*x[4]*x[5]^3 + 7*x[2]*x[4]*x[5]^3 + 13*x[3]*x[4]*x[5]^3 + 
>        7*x[4]^2*x[5]^3 + 4*x[3]*x[5]^4 + 2*x[4]*x[5]^4,
>    15*x[1]^3*x[2]^2 + 11*x[1]^2*x[2]^3 + 14*x[1]*x[2]^4 + x[2]^5 + 
>        2*x[1]^3*x[2]*x[3] + 11*x[1]*x[2]^3*x[3] + 7*x[2]^4*x[3] + 
>        3*x[1]^2*x[2]*x[3]^2 + 4*x[1]*x[2]^2*x[3]^2 + 13*x[2]^3*x[3]^2 + 
>        x[1]*x[2]*x[3]^3 + 4*x[2]*x[3]^4 + 16*x[1]^4*x[4] + 2*x[1]^3*x[2]*x[4] +
>        7*x[1]^2*x[2]^2*x[4] + 4*x[1]*x[2]^3*x[4] + 4*x[2]^4*x[4] + 
>        9*x[1]^3*x[3]*x[4] + x[1]^2*x[2]*x[3]*x[4] + 9*x[1]*x[2]^2*x[3]*x[4] + 
>        4*x[2]^3*x[3]*x[4] + 7*x[1]^2*x[3]^2*x[4] + 5*x[1]*x[2]*x[3]^2*x[4] + 
>        11*x[2]^2*x[3]^2*x[4] + 15*x[1]*x[3]^3*x[4] + 15*x[2]*x[3]^3*x[4] + 
>        x[3]^4*x[4] + 9*x[1]^2*x[2]*x[4]^2 + 16*x[1]*x[2]^2*x[4]^2 + 
>        9*x[2]^3*x[4]^2 + 3*x[1]^2*x[3]*x[4]^2 + 2*x[1]*x[2]*x[3]*x[4]^2 + 
>        14*x[2]^2*x[3]*x[4]^2 + 11*x[1]*x[3]^2*x[4]^2 + 16*x[2]*x[3]^2*x[4]^2 + 
>        4*x[3]^3*x[4]^2 + x[1]^2*x[4]^3 + 8*x[1]*x[2]*x[4]^3 + 14*x[2]^2*x[4]^3 
>        + 3*x[1]*x[3]*x[4]^3 + 16*x[2]*x[3]*x[4]^3 + 12*x[3]^2*x[4]^3 + 
>        7*x[1]*x[4]^4 + 5*x[2]*x[4]^4 + 4*x[3]*x[4]^4 + 2*x[4]^5 + 
>        8*x[1]^3*x[2]*x[5] + 5*x[1]^2*x[2]^2*x[5] + x[1]*x[2]^3*x[5] + 
>        16*x[2]^4*x[5] + 10*x[1]^2*x[2]*x[3]*x[5] + 12*x[1]*x[2]^2*x[3]*x[5] + 
>        9*x[2]^3*x[3]*x[5] + 15*x[1]*x[2]*x[3]^2*x[5] + 13*x[2]^2*x[3]^2*x[5] + 
>        4*x[2]*x[3]^3*x[5] + 8*x[1]^3*x[4]*x[5] + 16*x[1]^2*x[2]*x[4]*x[5] + 
>        11*x[1]*x[2]^2*x[4]*x[5] + 8*x[2]^3*x[4]*x[5] + 10*x[1]^2*x[3]*x[4]*x[5]
>        + 15*x[1]*x[2]*x[3]*x[4]*x[5] + 4*x[2]^2*x[3]*x[4]*x[5] + 
>        9*x[1]*x[3]^2*x[4]*x[5] + 16*x[2]*x[3]^2*x[4]*x[5] + 11*x[3]^3*x[4]*x[5]
>        + 4*x[1]^2*x[4]^2*x[5] + 6*x[1]*x[2]*x[4]^2*x[5] + 10*x[2]^2*x[4]^2*x[5]
>        + 11*x[1]*x[3]*x[4]^2*x[5] + 11*x[2]*x[3]*x[4]^2*x[5] + 
>        14*x[3]^2*x[4]^2*x[5] + 10*x[1]*x[4]^3*x[5] + 6*x[2]*x[4]^3*x[5] + 
>        5*x[3]*x[4]^3*x[5] + 4*x[4]^4*x[5] + 16*x[1]^2*x[2]*x[5]^2 + 
>        4*x[1]*x[2]^2*x[5]^2 + 11*x[2]^3*x[5]^2 + 9*x[1]*x[2]*x[3]*x[5]^2 + 
>        16*x[2]^2*x[3]*x[5]^2 + 8*x[2]*x[3]^2*x[5]^2 + 3*x[1]^2*x[4]*x[5]^2 + 
>        10*x[1]*x[2]*x[4]*x[5]^2 + 9*x[2]^2*x[4]*x[5]^2 + 
>        10*x[1]*x[3]*x[4]*x[5]^2 + 11*x[2]*x[3]*x[4]*x[5]^2 + 
>        11*x[3]^2*x[4]*x[5]^2 + 3*x[1]*x[4]^2*x[5]^2 + 14*x[2]*x[4]^2*x[5]^2 + 
>        7*x[3]*x[4]^2*x[5]^2 + 3*x[4]^3*x[5]^2 + x[1]*x[2]*x[5]^3 + 
>        15*x[2]^2*x[5]^3 + 10*x[2]*x[3]*x[5]^3 + x[1]*x[4]*x[5]^3 + 
>        2*x[2]*x[4]*x[5]^3 + 5*x[3]*x[4]*x[5]^3 + 6*x[4]^2*x[5]^3 + 
>        6*x[2]*x[5]^4 + 16*x[4]*x[5]^4,
>    2*x[1]^4*x[2] + 2*x[1]^3*x[2]^2 + 15*x[1]^2*x[2]^3 + x[1]*x[2]^4 + x[2]^5 + 
>        16*x[1]^4*x[3] + 8*x[1]^3*x[2]*x[3] + 7*x[1]^2*x[2]^2*x[3] + 
>        10*x[1]*x[2]^3*x[3] + 10*x[2]^4*x[3] + 10*x[1]^3*x[3]^2 + 
>        11*x[1]^2*x[2]*x[3]^2 + 9*x[1]*x[2]^2*x[3]^2 + 10*x[2]^3*x[3]^2 + 
>        7*x[1]^2*x[3]^3 + 10*x[1]*x[2]*x[3]^3 + 4*x[2]^2*x[3]^3 + 13*x[1]*x[3]^4
>        + 3*x[3]^5 + 2*x[1]^2*x[2]^2*x[4] + 11*x[2]^4*x[4] + 5*x[1]^3*x[3]*x[4] 
>        + 15*x[1]^2*x[2]*x[3]*x[4] + 16*x[1]*x[2]^2*x[3]*x[4] + 
>        4*x[2]^3*x[3]*x[4] + 16*x[1]^2*x[3]^2*x[4] + 12*x[1]*x[2]*x[3]^2*x[4] + 
>        4*x[2]^2*x[3]^2*x[4] + 15*x[1]*x[3]^3*x[4] + 14*x[2]*x[3]^3*x[4] + 
>        5*x[3]^4*x[4] + 13*x[1]^2*x[2]*x[4]^2 + 10*x[1]*x[2]^2*x[4]^2 + 
>        14*x[2]^3*x[4]^2 + 10*x[1]^2*x[3]*x[4]^2 + 9*x[1]*x[2]*x[3]*x[4]^2 + 
>        2*x[2]^2*x[3]*x[4]^2 + x[1]*x[3]^2*x[4]^2 + 15*x[2]*x[3]^2*x[4]^2 + 
>        15*x[3]^3*x[4]^2 + 2*x[1]*x[2]*x[4]^3 + 14*x[2]^2*x[4]^3 + 
>        15*x[1]*x[3]*x[4]^3 + 6*x[2]*x[3]*x[4]^3 + 3*x[3]^2*x[4]^3 + 
>        15*x[2]*x[4]^4 + 13*x[3]*x[4]^4 + 15*x[1]^3*x[2]*x[5] + 
>        11*x[1]^2*x[2]^2*x[5] + 16*x[1]*x[2]^3*x[5] + 10*x[2]^4*x[5] + 
>        4*x[1]^3*x[3]*x[5] + 10*x[1]^2*x[2]*x[3]*x[5] + 4*x[2]^3*x[3]*x[5] + 
>        3*x[1]^2*x[3]^2*x[5] + 6*x[1]*x[2]*x[3]^2*x[5] + 15*x[2]^2*x[3]^2*x[5] +
>        8*x[1]*x[3]^3*x[5] + 8*x[2]*x[3]^3*x[5] + x[3]^4*x[5] + 
>        13*x[1]^2*x[2]*x[4]*x[5] + 16*x[1]*x[2]^2*x[4]*x[5] + 8*x[2]^3*x[4]*x[5]
>        + 5*x[1]^2*x[3]*x[4]*x[5] + 16*x[1]*x[2]*x[3]*x[4]*x[5] + 
>        x[2]^2*x[3]*x[4]*x[5] + 10*x[1]*x[3]^2*x[4]*x[5] + 
>        9*x[2]*x[3]^2*x[4]*x[5] + 12*x[3]^3*x[4]*x[5] + 6*x[1]*x[2]*x[4]^2*x[5] 
>        + 6*x[2]^2*x[4]^2*x[5] + 12*x[1]*x[3]*x[4]^2*x[5] + 
>        16*x[2]*x[3]*x[4]^2*x[5] + 6*x[3]^2*x[4]^2*x[5] + 7*x[2]*x[4]^3*x[5] + 
>        15*x[3]*x[4]^3*x[5] + 7*x[1]^2*x[2]*x[5]^2 + 3*x[2]^3*x[5]^2 + 
>        3*x[1]^2*x[3]*x[5]^2 + 11*x[1]*x[2]*x[3]*x[5]^2 + x[2]^2*x[3]*x[5]^2 + 
>        12*x[1]*x[3]^2*x[5]^2 + 8*x[2]*x[3]^2*x[5]^2 + 4*x[3]^3*x[5]^2 + 
>        x[1]*x[2]*x[4]*x[5]^2 + 14*x[2]^2*x[4]*x[5]^2 + 8*x[1]*x[3]*x[4]*x[5]^2 
>        + 13*x[2]*x[3]*x[4]*x[5]^2 + 4*x[3]^2*x[4]*x[5]^2 + 
>        15*x[2]*x[4]^2*x[5]^2 + 16*x[1]*x[2]*x[5]^3 + 10*x[2]^2*x[5]^3 + 
>        9*x[1]*x[3]*x[5]^3 + 14*x[2]*x[3]*x[5]^3 + 12*x[3]^2*x[5]^3 + 
>        10*x[2]*x[4]*x[5]^3 + x[3]*x[4]*x[5]^3 + 15*x[2]*x[5]^4 + 9*x[3]*x[5]^4,
>    9*x[1]^4*x[2] + 14*x[1]^3*x[2]^2 + 5*x[1]^2*x[2]^3 + 2*x[1]*x[2]^4 + 
>        2*x[1]^3*x[2]*x[3] + 7*x[1]^2*x[2]^2*x[3] + 5*x[1]*x[2]^3*x[3] + 
>        7*x[2]^4*x[3] + 9*x[1]^2*x[2]*x[3]^2 + 12*x[1]*x[2]^2*x[3]^2 + 
>        2*x[2]^3*x[3]^2 + 9*x[1]*x[2]*x[3]^3 + 2*x[2]^2*x[3]^3 + x[2]*x[3]^4 + 
>        3*x[1]^3*x[2]*x[4] + 5*x[1]^2*x[2]^2*x[4] + 7*x[1]*x[2]^3*x[4] + 
>        13*x[2]^4*x[4] + 11*x[1]^2*x[2]*x[3]*x[4] + 4*x[1]*x[2]^2*x[3]*x[4] + 
>        11*x[2]^3*x[3]*x[4] + 14*x[1]*x[2]*x[3]^2*x[4] + 16*x[2]^2*x[3]^2*x[4] +
>        15*x[1]^2*x[2]*x[4]^2 + 11*x[1]*x[2]^2*x[4]^2 + 5*x[2]^3*x[4]^2 + 
>        6*x[1]*x[2]*x[3]*x[4]^2 + 9*x[2]^2*x[3]*x[4]^2 + 16*x[2]*x[3]^2*x[4]^2 +
>        9*x[2]^2*x[4]^3 + 15*x[2]*x[3]*x[4]^3 + 14*x[2]*x[4]^4 + 16*x[1]^4*x[5] 
>        + 16*x[1]^3*x[2]*x[5] + 16*x[1]^2*x[2]^2*x[5] + 3*x[1]*x[2]^3*x[5] + 
>        x[2]^4*x[5] + 9*x[1]^3*x[3]*x[5] + x[1]^2*x[2]*x[3]*x[5] + 
>        15*x[1]*x[2]^2*x[3]*x[5] + 10*x[2]^3*x[3]*x[5] + 7*x[1]^2*x[3]^2*x[5] + 
>        8*x[1]*x[2]*x[3]^2*x[5] + x[2]^2*x[3]^2*x[5] + 15*x[1]*x[3]^3*x[5] + 
>        5*x[2]*x[3]^3*x[5] + x[3]^4*x[5] + 11*x[1]^2*x[2]*x[4]*x[5] + 
>        14*x[1]*x[2]^2*x[4]*x[5] + 9*x[2]^3*x[4]*x[5] + 3*x[1]^2*x[3]*x[4]*x[5] 
>        + 11*x[1]*x[2]*x[3]*x[4]*x[5] + 14*x[2]^2*x[3]*x[4]*x[5] + 
>        11*x[1]*x[3]^2*x[4]*x[5] + 13*x[2]*x[3]^2*x[4]*x[5] + 4*x[3]^3*x[4]*x[5]
>        + x[1]^2*x[4]^2*x[5] + 5*x[1]*x[2]*x[4]^2*x[5] + 3*x[2]^2*x[4]^2*x[5] + 
>        3*x[1]*x[3]*x[4]^2*x[5] + 11*x[2]*x[3]*x[4]^2*x[5] + 
>        12*x[3]^2*x[4]^2*x[5] + 7*x[1]*x[4]^3*x[5] + 8*x[2]*x[4]^3*x[5] + 
>        4*x[3]*x[4]^3*x[5] + 2*x[4]^4*x[5] + 8*x[1]^3*x[5]^2 + 
>        6*x[1]^2*x[2]*x[5]^2 + 11*x[1]*x[2]^2*x[5]^2 + 14*x[2]^3*x[5]^2 + 
>        10*x[1]^2*x[3]*x[5]^2 + 14*x[1]*x[2]*x[3]*x[5]^2 + 2*x[2]^2*x[3]*x[5]^2 
>        + 9*x[1]*x[3]^2*x[5]^2 + 6*x[2]*x[3]^2*x[5]^2 + 11*x[3]^3*x[5]^2 + 
>        4*x[1]^2*x[4]*x[5]^2 + 12*x[1]*x[2]*x[4]*x[5]^2 + 13*x[2]^2*x[4]*x[5]^2 
>        + 11*x[1]*x[3]*x[4]*x[5]^2 + 10*x[2]*x[3]*x[4]*x[5]^2 + 
>        14*x[3]^2*x[4]*x[5]^2 + 10*x[1]*x[4]^2*x[5]^2 + 4*x[2]*x[4]^2*x[5]^2 + 
>        5*x[3]*x[4]^2*x[5]^2 + 4*x[4]^3*x[5]^2 + 3*x[1]^2*x[5]^3 + 
>        13*x[1]*x[2]*x[5]^3 + 6*x[2]^2*x[5]^3 + 10*x[1]*x[3]*x[5]^3 + 
>        2*x[2]*x[3]*x[5]^3 + 11*x[3]^2*x[5]^3 + 3*x[1]*x[4]*x[5]^3 + 
>        11*x[2]*x[4]*x[5]^3 + 7*x[3]*x[4]*x[5]^3 + 3*x[4]^2*x[5]^3 + x[1]*x[5]^4
>        + 9*x[2]*x[5]^4 + 5*x[3]*x[5]^4 + 6*x[4]*x[5]^4 + 16*x[5]^5,
>    13*x[1]^4*x[2] + 8*x[1]^3*x[2]^2 + 14*x[1]^2*x[2]^3 + 3*x[1]*x[2]^4 + 
>        11*x[2]^5 + 7*x[1]^3*x[2]*x[3] + 3*x[1]^2*x[2]^2*x[3] + 12*x[2]^4*x[3] +
>        3*x[1]^2*x[2]*x[3]^2 + 13*x[1]*x[2]^2*x[3]^2 + 3*x[2]^3*x[3]^2 + 
>        7*x[1]*x[2]*x[3]^3 + 2*x[2]^2*x[3]^3 + 7*x[2]*x[3]^4 + 
>        13*x[1]^3*x[2]*x[4] + 6*x[1]^2*x[2]^2*x[4] + 6*x[1]*x[2]^3*x[4] + 
>        6*x[2]^4*x[4] + 2*x[1]^2*x[2]*x[3]*x[4] + 15*x[1]*x[2]^2*x[3]*x[4] + 
>        14*x[2]^3*x[3]*x[4] + 3*x[1]*x[2]*x[3]^2*x[4] + 16*x[2]^2*x[3]^2*x[4] + 
>        3*x[2]*x[3]^3*x[4] + 6*x[1]^2*x[2]*x[4]^2 + 10*x[2]^3*x[4]^2 + 
>        7*x[2]^2*x[3]*x[4]^2 + 13*x[1]*x[2]*x[4]^3 + 5*x[2]^2*x[4]^3 + 
>        15*x[2]*x[3]*x[4]^3 + 13*x[2]*x[4]^4 + 16*x[1]^4*x[5] + 
>        5*x[1]^3*x[2]*x[5] + 11*x[1]^2*x[2]^2*x[5] + 3*x[1]*x[2]^3*x[5] + 
>        14*x[2]^4*x[5] + 10*x[1]^3*x[3]*x[5] + 2*x[1]^2*x[2]*x[3]*x[5] + 
>        14*x[1]*x[2]^2*x[3]*x[5] + 4*x[2]^3*x[3]*x[5] + 7*x[1]^2*x[3]^2*x[5] + 
>        10*x[1]*x[2]*x[3]^2*x[5] + 16*x[2]^2*x[3]^2*x[5] + 13*x[1]*x[3]^3*x[5] +
>        2*x[2]*x[3]^3*x[5] + 3*x[3]^4*x[5] + 5*x[1]^3*x[4]*x[5] + 
>        7*x[1]^2*x[2]*x[4]*x[5] + 8*x[1]*x[2]^2*x[4]*x[5] + 2*x[2]^3*x[4]*x[5] +
>        16*x[1]^2*x[3]*x[4]*x[5] + 9*x[1]*x[2]*x[3]*x[4]*x[5] + 
>        15*x[1]*x[3]^2*x[4]*x[5] + 3*x[2]*x[3]^2*x[4]*x[5] + 5*x[3]^3*x[4]*x[5] 
>        + 10*x[1]^2*x[4]^2*x[5] + 10*x[2]^2*x[4]^2*x[5] + x[1]*x[3]*x[4]^2*x[5] 
>        + x[2]*x[3]*x[4]^2*x[5] + 15*x[3]^2*x[4]^2*x[5] + 15*x[1]*x[4]^3*x[5] + 
>        14*x[2]*x[4]^3*x[5] + 3*x[3]*x[4]^3*x[5] + 13*x[4]^4*x[5] + 
>        4*x[1]^3*x[5]^2 + 13*x[1]^2*x[2]*x[5]^2 + 16*x[1]*x[2]^2*x[5]^2 + 
>        14*x[2]^3*x[5]^2 + 3*x[1]^2*x[3]*x[5]^2 + 16*x[1]*x[2]*x[3]*x[5]^2 + 
>        11*x[2]^2*x[3]*x[5]^2 + 8*x[1]*x[3]^2*x[5]^2 + 10*x[2]*x[3]^2*x[5]^2 + 
>        x[3]^3*x[5]^2 + 5*x[1]^2*x[4]*x[5]^2 + 15*x[1]*x[2]*x[4]*x[5]^2 + 
>        9*x[2]^2*x[4]*x[5]^2 + 10*x[1]*x[3]*x[4]*x[5]^2 + 
>        9*x[2]*x[3]*x[4]*x[5]^2 + 12*x[3]^2*x[4]*x[5]^2 + 12*x[1]*x[4]^2*x[5]^2 
>        + 3*x[2]*x[4]^2*x[5]^2 + 6*x[3]*x[4]^2*x[5]^2 + 15*x[4]^3*x[5]^2 + 
>        3*x[1]^2*x[5]^3 + 10*x[1]*x[2]*x[5]^3 + 14*x[2]^2*x[5]^3 + 
>        12*x[1]*x[3]*x[5]^3 + 6*x[2]*x[3]*x[5]^3 + 4*x[3]^2*x[5]^3 + 
>        8*x[1]*x[4]*x[5]^3 + 4*x[3]*x[4]*x[5]^3 + 9*x[1]*x[5]^4 + 14*x[2]*x[5]^4
>        + 12*x[3]*x[5]^4 + x[4]*x[5]^4 + 9*x[5]^5]);

We check a few of the invariants of X.

> Dimension(X);
2
> IsNonsingular(X);
true
> ArithmeticGenus(X);
0
> // get the sectional genus of X - ie the genus of a hyperplane section.
> ArithmeticGenus(X meet Scheme(P,P.1));
9

Now we get the canonical sheaf and hyperplane sheaf and check intersection numbers.

> KX := CanonicalSheaf(X);
> HX := StructureSheaf(X,1); // hyperplane sheaf
> IntersectionPairing(HX,HX); // should be 10 = Degree(X)
10
> Degree(X);
10
> IntersectionPairing(KX,HX); // should be 6
6
> IntersectionPairing(KX,KX); // should be -9 : lots of exceptional curves!
-9

We now get the adjunction map as a divisor map, compute its image X₁ and check some of the invariants of X₁ as well as its corresponding intersection numbers.

> mp1,X1 := DivisorMap(Twist(KX,1));
> Dimension(Ambient(X1)); Dimension(X1);
8
2
> KX1 := CanonicalSheaf(X1);
> HX1 := StructureSheaf(X1,1); // hyperplane sheaf of X1
> IntersectionPairing(HX1,HX1); // should be 13 = degree X1
13
> IntersectionPairing(KX1,HX1); // should be -3
-3
> IntersectionPairing(KX1,KX1); // should be -2 : fewer exceptional curves!
-2

We perform a second adjunction map to get X₂ and check it as above

> mp2,X2 := DivisorMap(Twist(KX1,1));
> Dimension(Ambient(X2)); Dimension(X2);
5
2
> KX2 := CanonicalSheaf(X2);
> HX2 := StructureSheaf(X2,1); // hyperplane sheaf X2
> IntersectionPairing(HX2,HX2); // = degree X2 = 5
5
> IntersectionPairing(KX2,HX2); // should be -5
-5
> IntersectionPairing(KX2,KX2); // should be 5
5

Now X₂ should be a degree 5 Del Pezzo with ( K)_X simeq OX( - 1). This last isomorphism can be verified by checking that there is a degree -2 isomorphism from ( K)_X to OX(1)! X₂ is much simpler than X: it is defined by 6 degree 2 polynomials.

> boo,d := IsIsomorphicWithTwist(KX2,HX2);
> boo; d;
true
-2
> X2;
Scheme over GF(17) defined by
y[3]^2*y[4] + 16*y[1]*y[4]^2 + 8*y[2]*y[4]^2 + 9*y[3]*y[4]^2 + 10*y[4]^3 + 
    3*y[3]^2*y[5] + 3*y[1]*y[4]*y[5] + 2*y[2]*y[4]*y[5] + 5*y[3]*y[4]*y[5] + 
    14*y[4]^2*y[5] + y[1]*y[5]^2 + 14*y[2]*y[5]^2 + y[4]*y[5]^2 + 6*y[5]^3 + 
    3*y[3]^2*y[6] + 15*y[1]*y[4]*y[6] + 2*y[2]*y[4]*y[6] + 9*y[3]*y[4]*y[6] + 
    3*y[4]^2*y[6] + 10*y[2]*y[5]*y[6] + y[3]*y[5]*y[6] + 6*y[4]*y[5]*y[6] + 
    7*y[5]^2*y[6] + 3*y[1]*y[6]^2 + 13*y[2]*y[6]^2 + 4*y[3]*y[6]^2 + 
    4*y[4]*y[6]^2 + 16*y[5]*y[6]^2 + 2*y[6]^3,
y[1]^2 + y[3]^2 + y[1]*y[4] + 15*y[2]*y[4] + 8*y[3]*y[4] + 6*y[4]^2 + 
    2*y[1]*y[5] + 12*y[2]*y[5] + y[3]*y[5] + 4*y[4]*y[5] + 4*y[5]^2 + 
    6*y[1]*y[6] + 10*y[2]*y[6] + 7*y[3]*y[6] + 7*y[5]*y[6] + 16*y[6]^2,
y[1]*y[2] + 13*y[3]^2 + 3*y[1]*y[4] + 14*y[2]*y[4] + 13*y[3]*y[4] + 5*y[4]^2 +
    14*y[1]*y[5] + 10*y[2]*y[5] + 2*y[3]*y[5] + 9*y[4]*y[5] + 6*y[5]^2 + 
    4*y[1]*y[6] + 13*y[2]*y[6] + 10*y[3]*y[6] + 3*y[4]*y[6] + y[5]*y[6] + 
    12*y[6]^2,
y[2]^2 + 16*y[3]^2 + 15*y[1]*y[4] + 3*y[3]*y[4] + y[4]^2 + 10*y[1]*y[5] + 
    12*y[2]*y[5] + 10*y[3]*y[5] + 11*y[4]*y[5] + 9*y[5]^2 + 5*y[1]*y[6] + 
    3*y[2]*y[6] + 2*y[3]*y[6] + 15*y[4]*y[6] + 12*y[5]*y[6] + 5*y[6]^2,
y[1]*y[3] + 13*y[3]^2 + y[1]*y[4] + 11*y[3]*y[4] + y[4]^2 + 16*y[1]*y[5] + 
    y[2]*y[5] + 15*y[3]*y[5] + 3*y[4]*y[5] + 7*y[1]*y[6] + 3*y[2]*y[6] + 
    9*y[3]*y[6] + 10*y[4]*y[6] + 8*y[5]*y[6] + 6*y[6]^2,
y[2]*y[3] + 16*y[3]^2 + 14*y[1]*y[4] + 3*y[2]*y[4] + y[3]*y[4] + y[4]^2 + 
    12*y[1]*y[5] + 9*y[3]*y[5] + 6*y[4]*y[5] + 2*y[5]^2 + 13*y[3]*y[6] + 
    9*y[4]*y[6] + 13*y[5]*y[6] + 12*y[6]^2

Finally we get the composed map from X to X₂ and check that it is (birationally) invertible.

> mp1r := Restriction(mp1,X,X1);
> mp2r := Restriction(mp2,X1,X2);
> mpc := Expand(mp1r*mp2r);
> boo := IsInvertible(mpc);
> boo;
true

Example `Sheaf_ell_crv_norm (H108E4)`

In this example, we show how the sheaf machinery can be effectively used as an alternative method to normalise the projective coordinate ring of a normal, but not projectively normal, projective variety. Here the coordinate ring is locally normal at all primes except the maximal homogeneous ideal.

Our example variety is C, an elliptic curve that has been embedded as a degree 8 subvariety of P³ over Q. C can be thought of as having been embedded in P⁷ by a complete linear system of degree 8 and then (isomorphically) projected down into P³. Such genus one curves embedded as degree 8 curves in P³ actually arise fairly naturally as models of homogeneous spaces arising in eight-descents.

We wish to recover the full embedding as a projective normal curve in P⁷. The coordinate ring of this is isomorphic to the normalisation of the coordinate ring of C in P³. From a sheaf-theoretic point of view, this is straightforward. The full embedding is the image of the divisor map corresponding to a hyperplane section of C or, equivalently, to the Serre twisting sheaf OX(1). The maximal module of OX(1) is isomorphic to the normalisation as an R-module, where R is the coordinate ring of C in P³, and it can be recovered as an algebra by taking the image of its associated divisor map. The global sections of OX(1) correspond to the full Riemann-Roch space of the divisor on the abstract curve given by a certain hyperplane divisor on C.

This example also illustrates another interesting point. In situations like these, the dimension of the full space of global sections of the Serre twisting sheaf can be computed from cohomology of the coordinate ring R. However, it is actually faster in this case to actually compute the full maximal module of OX(1), the zeroth graded part of this corresponding to the space of global sections and having the dimension of the zeroth cohomology group. In fact, though we only need to compute the dimension of this part, it is actually much quicker to compute the maximal module and compute cohomology of that than to compute the cohomology of the original defining module, which is R twisted once. This probably reflects to some extent that polynomial ring Groebner basis computations are much more highly tuned currently in Magma than the alternating algebra ones used in the cohomology computations. But the maximal module of a sheaf is generally a nicer object than a submodule with bits missing in the lower-graded pieces and has a smaller Castelnuevo-Mumford regularity etc. So, as we see in this example, it is often worth making sure that the maximal module of a sheaf is available before making cohomology calls.

> P<x,y,z,t> := ProjectiveSpace(Rationals(),3);
> C := Curve(P,[ x^2*y^2 - 23/59*x*y^3 + 9/59*y^4 + 27/59*x^3*z - 23/59*x^2*y*
> z - 6/59*x*y^2*z + 6/59*y^3*z - 10/59*x^2*z^2 + 5/59*x*y*z^2 - 3/59*y^2*z^2 +
> 1/59*x*z^3 - 74/59*x^3*t + 115/59*x^2*y*t - 83/59*x*y^2*t + 3/59*y^3*t -
> 105/59*x^2*z*t + 1/59*x*y*z*t - 2/59*y^2*z*t + 36/59*x*z^2*t + 4/59*y*z^2*t -
> 3/59*z^3*t + 297/59*x^2*t^2 - 135/59*x*y*t^2 + 52/59*y^2*t^2 + 68/59*x*z*t^2 -
> 11/59*y*z*t^2 - 18/59*z^2*t^2 - 315/59*x*t^3 + 42/59*y*t^3 + 96/59*t^4,
> x^3*y - 833/354*x*y^3 - 11/236*y^4 - 1633/708*x^3*z - 4675/708*x^2*y*z -
> 2633/708*x*y^2*z - 27/236*y^3*z + 805/354*x^2*z^2 + 223/59*x*y*z^2 -
> 4/59*y^2*z^2 - 38/59*x*z^3 + 3359/708*x^3*t + 3811/354*x^2*y*t +
> 1445/708*x*y^2*t + 303/118*y^3*t - 715/177*x^2*z*t - 527/177*x*y*z*t +
> 211/118*y^2*z*t + 347/354*x*z^2*t - 195/236*y*z^2*t - 4/59*z^3*t -
> 127/236*x^2*t^2 - 8237/708*x*y*t^2 + 65/708*y^2*t^2 + 1973/708*x*z*t^2 +
> 123/59*y*z*t^2 - 24/59*z^2*t^2 - 1753/354*x*t^3 + 873/236*y*t^3 + 128/59*t^4,
> x^4 + 269/354*x*y^3 + 35/236*y^4 + 1849/708*x^3*z + 4255/708*x^2*y*z -
> 247/708*x*y^2*z + 43/236*y^3*z - 727/354*x^2*z^2 - 82/59*x*y*z^2 +
> 2/59*y^2*z^2 + 19/59*x*z^3 - 5603/708*x^3*t - 3469/354*x^2*y*t -
> 1637/708*x*y^2*t - 63/118*y^3*t + 328/177*x^2*z*t - 769/177*x*y*z*t -
> 17/118*y^2*z*t + 151/354*x*z^2*t + 127/236*y*z^2*t + 2/59*z^3*t +
> 1391/236*x^2*t^2 + 7865/708*x*y*t^2 + 823/708*y^2*t^2 - 1901/708*x*z*t^2 +
> 86/59*y*z*t^2 + 12/59*z^2*t^2 + 493/354*x*t^3 - 761/236*y*t^3 - 64/59*t^4]);
> // get the hyperplane sheaf of C
> OC1 := StructureSheaf(C,1);
> // saturate to get the maximal module before making a cohomology call.
> //  It's much faster, though unnecessary here anyway!
> SaturateSheaf(~OC1);
> CohomologyDimension(OC1,0,0); //dimension of space of global sections = 8
8
> // get the projective normal embedding into P^7 and the image X
> //  should be defined by 20 quadrics.
> norm_mp,X := DivisorMap(OC1);
> ArithmeticGenus(X);
1
> B := MinimalBasis(Ideal(X));
> #B;
20
> [TotalDegree(f) : f in B];
[ 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2 ]

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Version: V2.16 of Mon Nov 16 15:04:45 EST 2009

MAGMA Computational Algebra System

Examples

Example Sheaf_big_ex (H108E3)

Example Sheaf_ell_crv_norm (H108E4)

Example `Sheaf_big_ex (H108E3)`

Example `Sheaf_ell_crv_norm (H108E4)`