Preprint 36/2017

When is a polynomial ideal binomial after an ambient automorphism?

Lukas Katthaen, Mateusz Michałek, and Ezra Miller

Contact the author: Please use for correspondence this email.
Submission date: 13. Jun. 2017
MSC-Numbers: 14Q99, 13P99, 14L30, 13A50, 14M25, 68W30
Link to arXiv:See the arXiv entry of this preprint.

Can an ideal I in a polynomial ring k[x] over a field be moved by a change of coordinates into a position where it is generated by binomials xaλxb with λ k, or by unital binomials (i.e., with λ = 0 or 1)? Can a variety be moved into a position where it is toric? By fibering the G-translates of I over an algebraic group G acting on affine space, these problems are special cases of questions about a family F of ideals over an arbitrary base B. The main results in this general setting are algorithms to find the locus of points in B over which the fiber of F

  • is contained in the fiber of a second family Fof ideals over B;
  • defines a variety of dimension at least d;
  • is generated by binomials; or
  • is generated by unital binomials.

A faster containment algorithm is also presented when the fibers of F are prime. The big-fiber algorithm is probabilistic but likely faster than known deterministic ones. Applications include the setting where a second group T acts on affine space, in addition to G, in which case algorithms compute the set of G-translates of I

  • whose stabilizer subgroups in T have maximal dimension; or
  • that admit a faithful multigrading by Zr of maximal rank r.

Even with no ambient group action given, the final application is an algorithm to

  • decide whether a normal projective variety is abstractly toric.

All of these loci in B and subsets of G are constructible; in some cases they are closed.

03.07.2017, 01:42