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Let $D$ be a digraph. A stable set $S$ of $D$ and a path partition $\mathcal{P}$ of $D$ are orthogonal if every path $P \in \mathcal{P}$ contains exactly one vertex of $S$. In 1982, Berge defined the class of $\alpha$-diperfect digraphs. A digraph $D$ is $\alpha$-diperfect if for every maximum stable set $S$ of $D$ there is a path partition $\mathcal{P}$ of $D$ orthogonal to $S$ and this property holds for every induced subdigraph of $D$. An anti-directed odd cycle is an orientation of an odd cycle $(x_0,\ldots,x_{2k},x_0)$ with $k\geq2$ in which each vertex $x_0,x_1,x_2,x_3,x_5,x_7\ldots,x_{2k-1}$ is either a source or a sink. Berge conjectured that a digraph $D$ is $\alpha$-diperfect if and only if $D$ does not contain an anti-directed odd cycle as an induced subdigraph. In this paper, we show that this conjecture is false by exhibiting an infinite family of orientations of complements of odd cycles with at least seven vertices that are not $\alpha$-diperfect.

Let $D$ be a digraph. A subset $S$ of $V(D)$ is a stable set if every pair of vertices in $S$ is non-adjacent in $D$. A collection of disjoint paths $\mathcal{P}$ of $D$ is a path partition of $V(D)$, if every vertex in $V(D)$ is exactly on a path of $\mathcal{P}$. We say that a stable set $S$ and a path partition $\mathcal{P}$ are orthogonal if each path of $P$ contains exactly one vertex of $S$. A digraph $D$ satisfies the $\alpha$-property if for every maximum stable set $S$ of $D$, there exists a path partition $\mathcal{P}$ such that $S$ and $\mathcal{P}$ are orthogonal. A digraph $D$ is $\alpha$-diperfect if every induced subdigraph of $D$ satisfies the $\alpha$-property. In 1982, Claude Berge proposed a characterization for $\alpha$-diperfect digraphs in terms of forbidden anti-directed odd cycles. In 2018, Sambinelli, Silva and Lee proposed a similar conjecture. A digraph $D$ satisfies the Begin-End-property or BE-property if for every maximum stable set $S$ of $D$, there exists a path partition $\mathcal{P}$ such that (i) $S$ and $\mathcal{P}$ are orthogonal and (ii) for each path $P \in \mathcal{P}$, either the start or the end of $P$ belongs to $S$. A digraph $D$ is BE-diperfect if every induced subdigraph of $D$ satisfies the BE-property. Sambinelli, Silva and Lee proposed a characterization for BE-diperfect digraphs in terms of forbidden blocking odd cycles. In this paper, we verified both conjectures for $3$-anti-circulant digraphs. We also present some structural results for $\alpha$-diperfect and BE-diperfect digraphs., Comment: arXiv admin note: substantial text overlap with arXiv:2111.12168

Let $D$ be a digraph. A subset $S$ of $V(D)$ is a stable set if every pair of vertices in $S$ is non-adjacent in $D$. A collection of disjoint paths $\mathcal{P}$ of $D$ is a path partition of $V(D)$, if every vertex in $V(D)$ is on a path of $\mathcal{P}$. We say that a stable set $S$ and a path partition $\mathcal{P}$ are orthogonal if each path of $P$ contains exactly one vertex of $S$. A digraph $D$ satisfies the $\alpha$-property if for every maximum stable set $S$ of $D$, there exists a path partition $\mathcal{P}$ such that $S$ and $\mathcal{P}$ are orthogonal. A digraph $D$ is $\alpha$-diperfect if every induced subdigraph of $D$ satisfies the $\alpha$-property. In 1982, Claude Berge proposed a characterization of $\alpha$-diperfect digraphs in terms of forbidden anti-directed odd cycles. In 2018, Sambinelli, Silva and Lee proposed a similar conjecture. A digraph $D$ satisfies the Begin-End-property or BE-property if for every maximum stable set $S$ of $D$, there exists a path partition $\mathcal{P}$ such that (i) $S$ and $\mathcal{P}$ are orthogonal and (ii) for each path $P \in \mathcal{P}$, either the start or the end of $P$ lies in $S$. A digraph $D$ is BE-diperfect if every induced subdigraph of $D$ satisfies the BE-property. Sambinelli, Silva and Lee proposed a characterization of BE-diperfect digraphs in terms of forbidden blocking odd cycles. In this paper, we show some structural results for $\alpha$-diperfect and BE-diperfect digraphs. In particular, we show that in every minimal counterexample $D$ to both conjectures, the size of a maximum stable set is smaller than $\vert V(D)\vert /2$. As an application we use these results to prove both conjectures for arc-locally in-semicomplete and arc-locally out-semicomplete digraphs.

Arc-locally semicomplete and arc-locally in-semicomplete digraphs were introduced by Bang-Jensen as a common generalization of both semicomplete and semicomplete bipartite digraphs in 1993. Later, Bang-Jensen (2004), Galeana-Sanchez and Goldfeder (2009) and Wang and Wang (2009) provided a characterization of strong arc-locally semicomplete digraphs. In 2009, Wang and Wang characterized strong arc-locally in-semicomplete digraphs. In 2012, Galeana-Sanchez and Goldfeder provided a characterization of all arc-locally semicomplete digraphs which generalizes some results by Bang-Jensen. In this paper, we characterize the structure of arbitrary connected arc-locally (out) in-semicomplete digraphs and arbitrary connected arc-locally semicomplete digraphs.

Let $D$ be a digraph. We call a subset $N$ of $V(D)$ $k$-independent if for every pair of vertices $u,v \in N$, $d(u,v) \geq k$; and we call it $\ell$-absorbent if for every vertex $u \in V(D) \setminus N$, there exists $v \in N$ such that $d(u,v) \leq \ell$. A $(k,\ell)$-kernel of $D$ is a subset of vertices which is $k$-independent and $\ell$-absorbent. A $k$-kernel is a $(k,k-1)$-kernel. In this report, we present the main results from our master's research regarding kernel theory. We prove that if a digraph $D$ is strongly connected and every cycle $C$ of $D$ satisfies: $(i)$ if $C \equiv 0 \pmod 3$, then $C$ has a short chord and $(ii)$ if $C \not \equiv 0 \pmod 3$, then $C$ has three short chords: two consecutive and a third crossing one of the former, then $D$ has a $3$-kernel. Moreover, we introduce a modification of the substitution method, proposed by Meyniel and Duchet in 1983, for $3$-kernels and use it to prove that a quasi-$3$-kernel-perfect digraph $D$ is $3$-kernel-perfect if every circuit of length not dividable by three has four short chords.

Let $D$ be a digraph. Given a set of vertices $S \subseteq V(D)$, an $S$-path partition $\mathcal{P}$ of $D$ is a collection of paths of $D$ such that $\{V(P) \colon P \in \mathcal{P}\}$ is a partition of $V(D)$ and $|V(P) \cap S| = 1$ for every $P \in \mathcal{P}$. We say that $D$ satisfies the $\alpha$-property if, for every maximum stable set $S$ of $D$, there exists an $S$-path partition of $D$, and we say that $D$ is $\alpha$-diperfect if every induced subdigraph of $D$ satisfies the $\alpha$-property. A digraph $C$ is an anti-directed odd cycle if (i) the underlying graph of $C$ is a cycle $x_1x_2 \cdots x_{2k + 1}x_1$, where $k \in \mathbb{Z}$ and $k \geq 2$, and (ii) each of the vertices $x_1, x_2, x_3, x_4, x_6,$ $x_8, \ldots, x_{2k}$ is either a source or a sink. Berge (1982) conjectured that a digraph is $\alpha$-diperfect if, and only if, it contains no induced anti-directed odd cycle. Remark that this conjecture is strikingly similar to Berge's conjecture on perfect graphs -- nowadays known as the Strong Perfect Graph Theorem (Chudnovsky, Robertson, Seymour, and Thomas, 2006). To the best of our knowledge, Berge's conjecture for $\alpha$-diperfect digraphs has been verified only for symmetric digraphs and digraphs whose underlying graph are perfect. In this paper, we verify it for digraphs whose underlying graphs are series-parallel and for in-semicomplete digraphs. Moreover, we propose a conjecture similar to Berge's and verify it for all the known cases of Berge's conjecture.

The crossing number of a graph $G$ is the least number of crossings over all possible drawings of $G$. We present a structural characterization of graphs with crossing number one.

The number $Z(n):=\lfloor n/2\rfloor\lfloor (n-1)/2\rfloor$ is the smallest number of crossings in a simple planar drawing of $K_{2,n}$ in which both vertices on the 2-side have the same clockwise rotation. For two vertices $u,v$ on the $q$-side of a simple drawing of $K_{p,q}$, let $\operatorname{cr}_D(u,v)$ denote the total number of crossings that edges incident with $u$ have with edges incident with $v$. We show that in any simple drawing $D$ of $K_{p,q}$ in a surface $\Sigma$ the number of pairs of vertices on the $q$-side of $K_{p,q}$ having $\operatorname{cr}_D(u,v)

Let $k$ be a positive integer and let $D$ be a digraph. A path partition $\sP$ of $D$ is a set of vertex-disjoint paths which covers $V(D)$. Its $k$-norm is defined as $\sum_{P \in \sP} \Min{|V(P)|, k}$. A path partition is $k$-optimal if its $k$-norm is minimum among all path partitions of $D$. A partial $k$-coloring is a collection of $k$ disjoint stable sets. A partial $k$-coloring $\sC$ is orthogonal to a path partition $\sP$ if each path $P \in \sP$ meets $\min\{|P|,k\}$ distinct sets of $\sC$. Berge (1982) conjectured that every $k$-optimal path partition of $D$ has a partial $k$-coloring orthogonal to it. A (path) $k$-pack of $D$ is a collection of at most $k$ vertex-disjoint paths in $D$. Its weight is the number of vertices it covers. A $k$-pack is optimal if its weight is maximum among all $k$-packs of $D$. A coloring of $D$ is a partition of $V(D)$ into stable sets. A $k$-pack $\sP$ is orthogonal to a coloring $\sC$ if each set $C \in \sC$ meets $\Min{|C|, k}$ paths of $\sP$. Aharoni, Hartman and Hoffman (1985) conjectured that every optimal $k$-pack of $D$ has a coloring orthogonal to it. A digraph $D$ is semicomplete if every pair of distinct vertices of $D$ is adjacent. A digraph $D$ is locally in-semicomplete if, for every vertex $v \in V(D)$, the in-neighborhood of $v$ induces a semicomplete digraph. Locally out-semicomplete digraphs are defined similarly. In this paper, we prove Berge's and Aharoni-Hartman-Hoffman's Conjectures for locally in/out-semicomplete digraphs.

Let $G=(V,E)$ be an $n$-vertex graph with $m$ edges. A function $f : V \cup E \rightarrow \{1, \ldots, n+m\}$ is an edge-magic labeling of $G$ if $f$ is bijective and, for some integer $k$, we have $f(u)+f(v)+f(uv) = k$ for every edge $uv \in E$. Furthermore, if $f(V) = \{1, \ldots, n\}$, then we say that $f$ is a super edge-magic labeling. A constellation, which is a collection of stars, is symmetric if the number of stars of each size is even except for at most one size. We prove that every symmetric constellation with an odd number of stars admits a super edge-magic labeling. We say that a caterpillar is of type $(r,s)$ if $r$ and $s$ are the sizes of its parts, where $r \leq s$. We also prove that every collection with an odd number of same-type caterpillars admits an edge-magic labeling.

A path (resp. cycle) decomposition of a graph $G$ is a set of edge-disjoint paths (resp. cycles) of $G$ that covers the edge set of $G$. Gallai (1966) conjectured that every graph on $n$ vertices admits a path decomposition of size at most $\lfloor (n+1)/2\rfloor$, and Haj\'os (1968) conjectured that every Eulerian graph on $n$ vertices admits a cycle decomposition of size at most $\lfloor (n-1)/2\rfloor$. Gallai's Conjecture was verified for many classes of graphs. In particular, Lov\'asz (1968) verified this conjecture for graphs with at most one vertex of even degree, and Pyber (1996) verified it for graphs in which every cycle contains a vertex of odd degree. Haj\'os' Conjecture, on the other hand, was verified only for graphs with maximum degree $4$ and for planar graphs. In this paper, we verify Gallai's and Haj\'os' Conjectures for graphs with treewidth at most $3$. Moreover, we show that the only graphs with treewidth at most $3$ that do not admit a path decomposition of size at most $\lfloor n/2\rfloor$ are isomorphic to $K_3$ or $K_5-e$. Finally, we use the technique developed in this paper to present new proofs for Gallai's and Haj\'os' Conjectures for graphs with maximum degree at most $4$, and for planar graphs with girth at least $6$.

In this paper we study the facility leasing problem with penalties. We present a primal-dual algorithm which is a 3-approximation, based on the algorithm by Nagarajan and Williamson for the facility leasing problem and on the algorithm by Charikar et al. for the facility location problem with penalties.

In this paper we introduce a superclass of split digraphs, which we call spine digraphs. Those are the digraphs D whose vertex set can be partitioned into two sets X and Y such that the subdigraph induced by X is traceable and Y is a stable set. We also show that Linial's Conjecture holds for spine digraphs.

We consider questions related to the existence of spanning trees in graphs with the property that after the removal of any path in the tree the graph remains connected. We show that, for planar graphs, the existence of trees with this property is closely related to the Hamiltonicity of the graph. For graphs with a 1- or 2-vertex cut, the Hamiltonicity also plays a central role. We also deal with spanning trees satisfying this property restricted to paths arising from fundamental cycles. The cycle space of a graph can be generated by the fundamental cycles of any spanning tree, and Tutte showed, that for a 3-connected graph, it can be generated by nonseparating cycles. We are also interested in the existence of a fundamental basis consisting of nonseparating cycles., Comment: 10 pages

We consider the problem of finding an integral packing of branchings in a capacitated digraph with root-set demands. Schrijver described an algorithm that returns a packing with at most m+n^3+r branchings that makes at most m(m+n^3+r) calls to an oracle that basically computes a minimum cut, where n is the number of vertices, m is the number of arcs and r is the number of root-sets of the input digraph. In this work we provide an algorithm, inspired on ideas of Schrijver and on an paper of Gabow and Manu, that returns a packing with at most m+r-1 branchings and makes at most 2n+m+r-1 oracle calls., Comment: We are fixing a flaw in the proof