On determining when small embeddings of partial Steiner triple systems exist

TL;DR

This paper investigates the existence of small embeddings of partial Steiner triple systems, extending NP-completeness results and providing counterexamples to a conjecture about their existence.

Contribution

It extends Colbourn's NP-completeness result to small embeddings and presents counterexamples to a conjecture on their existence.

Findings

Small embeddings are NP-complete to determine.

Counterexamples disprove a conjecture on small embedding existence.

Abstract

A partial Steiner triple system of order $u$ is a pair $(U,\mathcal{A})$ where $U$ is a set of $u$ elements and $\mathcal{A}$ is a set of triples of elements of $U$ such that any two elements of $U$ occur together in at most one triple. If each pair of elements occur together in exactly one triple it is a Steiner triple system. An embedding of a partial Steiner triple system $(U,\mathcal{A})$ is a (complete) Steiner triple system $(V,\mathcal{B})$ such that $U \subseteq V$ and $\mathcal{A} \subseteq \mathcal{B}$. For a given partial Steiner triple system of order $u$ it is known that an embedding of order $v \geq 2u+1$ exists whenever $v$ satisfies the obvious necessary conditions. Determining whether "small" embeddings of order $v < 2u+1$ exist is a more difficult task. Here we extend a result of Colbourn on the $\mathsf{NP}$-completeness of these problems. We also exhibit a family of counterexamples to a conjecture concerning when small embeddings exist.