Lower General Position in Cartesian Products

A subset <i>S</i> of vertices of a graph <i>G</i> is in general position if no shortest path in <i>G</i> contains three vertices of <i>S</i>. The <i>general position problem</i> consists of finding the number of vertices in a largest general position set of <i>G</i>, whilst the <i>lower general position</i> problem asks for a smallest maximal general position set. In this paper we determine the lower general position numbers of several families of Cartesian products. We also show that the existence of small maximal general position sets in a Cartesian product is connected to a special type of general position set in the factors, which we call a <i>terminal</i> set, for which adding any vertex <i>u</i> from outside the set creates three vertices in a line with <i>u</i> as an endpoint. We give a constructive proof of the existence of terminal sets for graphs with diameter at most three. We also present conjectures on the existence of terminal sets for all graphs and a lower bound on the lower general position number of a Cartesian product in terms of the lower general position numbers of its factors.