# Do subsets of generators of a toric ideal generate a toric ideal?

Given a toric ideal, say $J$, in a polynomial ring $k[x_1,...,x_n]$ we can find a finite generating set for $J$. Is it possible, perhaps under additional assumptions on the structure of $J$, to give a finite minimal generating set for $J$ such that every subset of generators also generates a toric ideal.

If not, are there any known counterexamples in the general case?

If yes, could you provide a reference? Does it generalize to lattice ideals?

Motivation: For particularly chosen, generating sets of toric ideals there exist subsets that also generate a toric ideal. For e.g. the 2xn determinantal ideal is toric. A generating set is given by the 2x2 minors of the defining matrix, say $M$. Then the ideal generated by those minors which correspond to a subset of the columns of the $M$ is also toric.

-

I believe this is a counterexample. Toric ideals are prime by definition (assuming that I am remembering correctly). Then I think that the ideal of the twisted cubic $C\subseteq \mathbb P^3$ ought to do it.

$J=\langle xz-y^2, xw-yz,wy-z^2\rangle$.

Any two of the three generators will intersect in the union of $C$ and a line. For instance, the vanishing of $I=\langle xz-y^2, xw-yz\rangle$ is $C\cup L$ where $L=V(x,y)$. So $I$ can't be prime.

-
Thanks for that example Daniel. I have seen examples like these where a particularly chosen generating set does not have the sought property (subsets of the generating set generating toric ideals). However, I am not sure if it's easy to show, even in the particular case of this example, whether no such minimal generating set exists. –  Timothy Wagner Nov 12 '10 at 6:11
Rephrasing the above argument: any two of any minimal set of generators will cut out a scheme of dimension 1. It must strictly contain the twisted cubic, since the ideal of the twisted cubic requires three generators. –  Vivek Shende Nov 12 '10 at 7:41
@Timothy: Vivek's comment says it perfectly. I'm curious what led you to this question, by the way. –  Daniel Erman Nov 12 '10 at 18:43
Thanks Vivek and Daniel for clearing that up. @Daniel, I have proved a result on the integral closure of certain products of ideals. I have also shown that it holds for certain kinds of toric ideals and it will hold for toric ideals satisfying the above property. I just wished to know if something like this was already known for toric ideals. I actually only need that the subsets of generators generate an integrally closed ideal. But integral closure is less tractable, so I decided to first see if the above holds. I am sorry I cannot go into more detail since I am yet to write up the article. –  Timothy Wagner Nov 12 '10 at 20:53
What is going on can be explained completely in terms of exponent vectors. To do so we can map each generator $x^u -x^v$ of $J$ to the exponent vector $u-v \in \mathbb{Z}^n$. Conversely, for each integer vector $s\in \mathbb{Z}^n$ we can form a binomial $x^{s^+} - x^{s^-}$ where $s^\pm_i = \max(\pm s_i, 0)$ are the positive and negative part of $s$, such that $s = s^+ - s^-$. Now, take the integer vector images of a minimal generating set of $J$ and let $L_J$ be the sublattice of $\mathbb{Z}^n$ that they generate. This lattice is independent of the chosen generators and we have the following basic fact:
Prop. Let $S \subset L$ be any finite set of vectors spanning $L$ as a lattice. Then $J = \langle x^{s^+} - x^{s^-} : s\in S\rangle : \left(\prod_i x_i\right)^\infty$.
Typically a generating set of $L$ will be much smaller than any image of a minimal generating set of $J$. (Google for "Markov basis" to find tons of examples from algebraic statistics.) The proposition says that $J$ can be reconstructed from $L$, but also that whenever you remove a generator from $J$ and $L$ does not change, then the resulting ideal can not be prime (it has some extra components that are contained in the union of the coordinate planes). Dan's example is of this form.
Therefore the only chance that you may have, is when $J$ is, in fact, a complete intersection. This case is treated in the paper "Affine semigroup rings that are complete intersections" by Fischer, Morris, and Shapiro Proc. AMS 125 (11), 1997. It contains a combinatorial characterization complete intersection semigroups in Theorem 3.1 which may be useful for finding a counterexample in that case too.