For an end $\tau $ and a tree $T$ of a graph $G$ we denote respectively by $m(\tau )$ and $m_{T}(\tau )$ the maximum numbers of pairwise disjoint rays of $G$ and $T$ belonging to $\tau $, and we define $\mathop {\mathrm tm}(\tau ) := \min \lbrace m_{T}(\tau )\: T \text{is} \text{a} \text{spanning} \text{tree} \text{of} G \rbrace $. In this paper we give partial answers—affirmative and negative ones—to the general problem of determining if, for a function $f$ mapping every end $\tau $ of $G$ to a cardinal $f(\tau )$ such that $\mathop {\mathrm tm}(\tau ) \le f(\tau ) \le m(\tau )$, there exists a spanning tree $T$ of $G$ such that $m_{T}(\tau ) = f(\tau )$ for every end $\tau $ of $G$.
The notion of travel groupoids was introduced by L. Nebeský in 2006 in connection with a study on geodetic graphs. A travel groupoid is a pair of a set $V$ and a binary operation $*$ on $V$ satisfying two axioms. We can associate a graph with a travel groupoid. We say that a graph $G$ has a travel groupoid if the graph associated with the travel groupoid is equal to $G$. Nebeský gave a characterization of finite graphs having a travel groupoid. In this paper, we study travel groupoids on infinite graphs. We answer a question posed by Nebeský, and we also give a characterization of infinite graphs having a travel groupoid.