\documentclass[12pt]{article}
\usepackage{amsmath,amssymb,amsfonts}
\begin{document}
The concept of defining set has been studied in block designs and,
under the name critical sets, in Latin squares and Room squares.
Here we study defining sets for directed designs. A
$t$-$(v,k,\lambda)$ directed design (DD) is a pair $(V,{\cal B})$,
where $V$ is a $v$-set and ${\cal B}$ is a collection of ordered
blocks (or $k$-tuples of $V$), for which each $t$-tuple of $V$
appears in precisely $\lambda$ blocks. A set of blocks which is a
subset of a unique $t$-$(v,k,\lambda)$ DD is said to be a {\it
defining set} of the directed design.\\ As in the case of block
designs, finding defining sets seems to be a difficult problem. In
this note we introduce some lower bounds for the number of blocks
in smallest defining sets in directed designs, determine the
precise number of blocks in smallest defining sets for some
directed designs with small parameters and point out an open
problem relating to the number of blocks needed to define a
directed design as compared with the number needed to define its
underlying undirected design.
\end{document}