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We prove that for every set $S$ of vertices of a directed graph $D$, the maximum number of vertices in $S$ contained in a collection of vertex-disjoint cycles in $D$ is at least the minimum size of a set of vertices that hits all cycles containing a vertex of $S$. As a consequence, the directed tree-width of a directed graph is linearly bounded in its cycle-width, which improves the previously known quadratic upper bound. We further show that the corresponding statement in bidirected graphs is true and that its edge-variant holds in both undirected and directed graphs, but fails in bidirected graphs. The vertex-version in undirected graphs remains an open problem., Comment: 12 pages, 2 figures

Halin showed that any thick end of any graph contains an infinite grid. We extend Halin's theorem to digraphs. More precisely, we show that for any infinite family $\mathcal{R}$ of disjoint equivalent out-rays there is a grid whose vertical rays are contained in $\mathcal{R}$. Furthermore, we obtain similar results for in-rays and necklaces., Comment: 18 pages, 5 figures

We prove that there exist four operations such that given any two strongly $2$-connected digraphs $H$ and $D$ where $H$ is a butterfly-minor of $D$, there exists a sequence $D_0,\dots, D_n$ where $D_0=H$, $D_n=D$ and for every $0\leq i\leq n-1$, $D_i$ is a strongly $2$-connected butterfly-minor of $D_{i+1}$ which is obtained by a single application of one of the four operations. As a consequence of this theorem, we obtain that every strongly $2$-connected digraph can be generated from a concise family of strongly $2$-connected digraphs by using these four operations., Comment: 42 pages

In this paper we investigate normal trees of directed graphs, which extend the fundamental concept of normal trees of undirected graphs and form a special case of normal trees of connectoids. We show that a directed graph $D$ has a normal spanning tree if and only if the topological space $|D|$ is metrizable. This generalises Diestel's result for undirected graphs. We further show that the existence of normal arborescences implies the existence of normal trees in directed graphs and the converse does not hold true in general. Furthermore, we discuss properties of normal trees that carry over from connectoids to directed graphs., Comment: 14 pages

The star-comb lemma is a fundamental tool in infinite graph theory, which states that for any infinite set $U$ of vertices in a connected graph $G$ there exists either a subdivided infinite star in $G$ with all leaves in $U$, or an infinite comb in $G$ with all teeth in $U$. In this paper, we elaborate a pendant of the star-comb lemma for directed graphs. More precisely, we prove that for any infinite set $U$ of vertices in a strongly connected directed graph $D$, there exists a strongly connected substructure of $D$ attached to infinitely many vertices of $U$ that is either shaped by a star or shaped by a comb, or is a chain of triangles., Comment: 20 pages, 16 figures. arXiv admin note: text overlap with arXiv:2406.03883

It is well-known that for any set $U$ of vertices in a connected graph $G$ there is either a subdivided star in $G$ with a large number of leaves in $U$, or a comb in $G$ with a large number of teeth in $U$. In this paper we extend this property to directed graphs. More precisely, we prove that for any set $U$ of vertices in a strongly connected directed graph $D$, there exists a strongly connected substructure of $D$ attached to a large number of vertices in $U$ that is either shaped by a star or shaped by a comb., Comment: 15 pages, 11 figures

In this series we introduce and investigate the concept of connectoids, which captures the connectivity structure of various discrete objects like undirected graphs, directed graphs, bidirected graphs, hypergraphs or finitary matroids. In the first paper of this series we developed a universal end space theory based on connectoids that unifies the existing end spaces of undirected and directed graphs. Further, we introduced a notion of normal trees of connectoids which represents the ends of connectoids as normal trees do in undirected graphs. In this paper we show that our notion of normal trees generalises normal trees of undirected graphs to connectoids by characterising their existence: We extend Jung's famous characterisation via dispersed sets to connectoids, and show that normal spanning trees exist if they exist in some neighbourhood of each end. Furthermore, we show that a connectoid has a normal spanning tree if and only if its groundset can be well-ordered in a certain way, called countable separation number. In this way we establish a unified notion of normal trees for undirected, directed and bidirected graphs, as well as for hypergraphs and finitary matroids., Comment: 33 pages, 1 figure

In this series we introduce and investigate the concept of connectoids, which captures the connectivity structure of various discrete objects like undirected graphs, directed graphs, bidirected graphs, hypergraphs or finitary matroids. In this paper we develop a universal end space theory based on connectoids: the end spaces of connectoids unify the existing end spaces of undirected and directed graphs and establish end spaces for bidirected graphs, hypergraphs and finitary matroids. The main result shows that the tangle-like description of ends in undirected graphs, called directions, extends to connectoids: there is a one-to-one correspondence between the directions of a connectoid and its ends. Furthermore, we generalise normal trees of undirected graphs to connectoids and show that normal trees represent the ends of a connectoid as they do for undirected graphs., Comment: 21 pages, 1 figure

A digraph $H$ is called ubiquitous if every digraph that contains arbitrarily many vertex-disjoint copies of $H$ also contains infinitely many vertex-disjoint copies of $H$. We study oriented double rays, that is, digraphs $H$ whose underlying undirected graphs are double rays. Calling a vertex of an oriented double ray a turn if it has in-degree or out-degree 2, we prove that an oriented double ray with at least one turn is ubiquitous if and only if it has a (finite) odd number of turns. It remains an open problem to determine whether the consistently oriented double ray is ubiquitous., Comment: 11 pages, 1 figure, minor changes

Bidirected graphs are a generalisation of directed graphs that arises in the study of undirected graphs with perfect matchings. Menger's famous theorem - the minimum size of a set separating two vertex sets $X$ and $Y$ is the same as the maximum number of disjoint paths connecting them - is generally not true in bidirected graphs. We introduce a sufficient condition for $X$ and $Y$ which yields a version of Menger's Theorem in bidirected graphs that in particular implies its directed counterpart., Comment: 23 pages, 6 figures

We introduce torsoids, a canonical structure in matching covered graphs, corresponding to the bricks and braces of the graph. This allows a more fine-grained understanding of the structure of finite and infinite directed graphs with respect to their 1-separations.

Call a digraph $H$ \emph{ubiquitous} if every digraph $D$ that contains $k$ vertex-disjoint copies of $H$ for every $k \in \mathbb{N}$ also contains infinitely many vertex-disjoint copies of $H$. We characterise which digraphs whose underlying undirected graph is a ray are ubiquitous., Comment: 12 pages, 3 figures