English

A polynomial ideal associated to any $t$-$(v,k,\lambda)$ design

Combinatorics 2018-03-14 v1

Abstract

We consider ordered pairs (X,B)(X,\mathcal{B}) where XX is a finite set of size vv and B\mathcal{B} is some collection of kk-element subsets of XX such that every tt-element subset of XX is contained in exactly λ\lambda "blocks" BBB\in \mathcal{B} for some fixed λ\lambda. We represent each block BB by a zero-one vector cB\mathbf{c}_B of length vv and explore the ideal I(B)\mathcal{I}(\mathcal{B}) of polynomials in vv variables with complex coefficients which vanish on the set {cBBB}\{ \mathbf{c}_B \mid B \in \mathcal{B}\}. After setting up the basic theory, we investigate two parameters related to this ideal: γ1(B)\gamma_1(\mathcal{B}) is the smallest degree of a non-trivial polynomial in the ideal I(B)\mathcal{I}(\mathcal{B}) and γ2(B)\gamma_2(\mathcal{B}) is the smallest integer ss such that I(B)\mathcal{I}(\mathcal{B}) is generated by a set of polynomials of degree at most ss. We first prove the general bounds t/2<γ1(B)γ2(B)kt/2 < \gamma_1(\mathcal{B}) \le \gamma_2(\mathcal{B}) \le k. Examining important families of examples, we find that, for symmetric 2-designs and Steiner systems, we have γ2(B)t\gamma_2(\mathcal{B}) \le t. But we expect γ2(B)\gamma_2(\mathcal{B}) to be closer to kk for less structured designs and we indicate this by constructing infinitely many triple systems satisfying γ2(B)=k\gamma_2(\mathcal{B})=k.

Keywords

Cite

@article{arxiv.1803.04931,
  title  = {A polynomial ideal associated to any $t$-$(v,k,\lambda)$ design},
  author = {William J. Martin and Douglas R. Stinson},
  journal= {arXiv preprint arXiv:1803.04931},
  year   = {2018}
}

Comments

26 pages, 1 figure

R2 v1 2026-06-23T00:51:57.197Z