3 research outputs found
Block-avoiding point sequencings
Let and be positive integers. Recent papers by Kreher, Stinson and
Veitch have explored variants of the problem of ordering the points in a triple
system (such as a Steiner triple system, directed triple system or Mendelsohn
triple system) on points so that no block occurs in a segment of
consecutive entries (thus the ordering is locally block-avoiding). We describe
a greedy algorithm which shows that such an ordering exists, provided that
is sufficiently large when compared to . This algorithm leads to improved
bounds on the number of points in cases where this was known, but also extends
the results to a significantly more general setting (which includes, for
example, orderings that avoid the blocks of a design). Similar results for a
cyclic variant of this situation are also established.
We construct Steiner triple systems and quadruple systems where can be
large, showing that a bound of Stinson and Veitch is reasonable. Moreover, we
generalise the Stinson--Veitch bound to a wider class of block designs and to
the cyclic case.
The results of Kreher, Stinson and Veitch were originally inspired by results
of Alspach, Kreher and Pastine, who (motivated by zero-sum avoiding sequences
in abelian groups) were interested in orderings of points in a partial Steiner
triple system where no segment is a union of disjoint blocks. Alspach~\emph{et
al.}\ show that, when the system contains at most pairwise disjoint blocks,
an ordering exists when the number of points is more than . By making
use of a greedy approach, the paper improves this bound to .Comment: 38 pages. Typo in the statement of Theorem 17 corrected, and other
minor changes mad