We provide two ways to show that the R. Thompson group $F$ has maximal subgroups of infinite index which do not fix any number in the unit interval under the natural action of $F$ on $(0,1)$, thus solving a problem by D. Savchuk. The first way employs Jones' subgroup of the R. Thompson group $F$ and leads to an explicit finitely generated example. The second way employs directed 2-complexes and 2-dimensional analogs of Stallings' core graphs, and gives many implicit examples. We also show that $F$ has a decreasing sequence of finitely generated subgroups $F>H_1>H_2>...$ such that $\cap H_i={1}$ and for every $i$ there exist only finitely many subgroups of $F$ containing $H_i$.[-]

Excursions are walks which start and end at prescribed locations. In this talk we consider the counting sequences of excursions, more precisely, the functional equations their generating functions satisfy. We focus on two sources of excursion problems: walks defined by their allowable steps, taken on integer lattices restricted to cones; and walks on Cayley graphs with a given set of generators. The latter is related to the cogrowth problems of groups. In both cases we are interested in relating the nature of the generating function (i.e. rational, algebraic, D-finite, etc.) and combinatorial properties of the models. We are also interested in the relation between the excursions, and less restricted families of walks.
Please note: A few corrections were made to the PDF file of this talk, the new version is available at the bottom of the page.[-]