English

Equigenerated Gorenstein ideals of codimension three

Commutative Algebra 2021-07-13 v4

Abstract

We focus on the structure of a homogeneous Gorenstein ideal II of codimension three in a standard polynomial ring R=\kk[x1,,xn]R=\kk[x_1,\ldots,x_n] over a field \kk\kk, assuming that II is generated in a fixed degree dd. For such an ideal II this degree comes along with the minimal number of generators of II and the degree of the entries of the associated skew-symmetric matrix in a simple formula. We give an elementary characteristic-free argument to the effect that, for any such data linked by this formula, there exists a Gorenstein ideal II of codimension three filling them. We conjecture that, for arbitrary n2n\geq 2, an ideal I\kk[x1,,xn]I\subset \kk[x_1,\ldots,x_n] generated by a general set of rn+2r\geq n+2 forms of degree d2d\geq 2 is Gorenstein if and only if d=2d=2 and r=(n+12)1r= {{n+1}\choose 2}-1. We prove the `only if' implication of this conjecture when n=3n=3. For arbitrary n2n\geq 2, we prove that if d=2d=2 and r(n+2)(n+1)/6r\geq (n+2)(n+1)/6 then the ideal is Gorenstein if and only if r=(n+12)1r={{n+1}\choose 2}-1, which settles the `if' assertion of the conjecture for n5n\leq 5. Finally, we elaborate around one of the questions of Fr\"oberg--Lundqvist. In a different direction, we reveal a connection between the Macaulay inverse and the so-called Newton dual, a matter so far not brought out to our knowledge. Finally, we consider the question as to when the link (1m,,nm):f(\ell_1^m,\ldots,\ell_n^m):\mathfrak{f} is equigenerated, where 1,,n\ell_1,\ldots,\ell_n are independent linear forms and f\mathfrak{f} is a form, is given a solution in some important cases.

Keywords

Cite

@article{arxiv.2005.12953,
  title  = {Equigenerated Gorenstein ideals of codimension three},
  author = {Dayane Lira and Zaqueu Ramos and Aron Simis},
  journal= {arXiv preprint arXiv:2005.12953},
  year   = {2021}
}