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Keywords: biological data analysis, graph theoretical algorithms, thresholding algorithms, gene co-expression
Published in DiRROS: 08.05.2026; Views: 88; Downloads: 53
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Abstract: Given a graph $G$, a subset $M\subseteq V(G)$ is a mutual-visibility (MV) set if for every $u,v\in M$, there exists a $u,v$-geodesic whose internal vertices are not in $M$. We investigate proper vertex colorings of graphs whose color classes are mutual-visibility sets. The main concepts that arise in this investigation are independent mutual-visibility (IMV) sets and vertex partitions into these sets (IMV colorings). The IMV number $\mu_{i}$ and the IMV chromatic number $\chi_{\mu_{i}}$ are defined as maximum and minimum cardinality taken over all IMV sets and IMV colorings, respectively. Along the way, we also continue with the study of MV chromatic number $\chi_{\mu}$ (as the smallest number of sets in a vertex partition into MV sets), which was initiated in an earlier paper. We establish a close connection between the (I)MV chromatic numbers of subdivisions of complete graphs and Ramsey numbers $R(4^k;2)$. From the computational point of view, we prove that the problems of computing $\chi_{\mu_{i}}$ and $\mu_{i}$ are NP-complete, and that it is NP-hard to decide whether a graph $G$ satisfies $\mu_i(G)=\alpha(G)$ where $\alpha(G)$ is the independence number of $G$. Several tight bounds on $\chi_{\mu_{i}}$, $\chi_{\mu}$ and $\mu_{i}$ are given. Exact values/formulas for these parameters in some classical families of graphs are proved. In particular, we prove that $\chi_{\mu_{i}}(T)=\chi_{\mu}(T)$ holds for any tree $T$ of order at least $3$, and determine their exact formulas in the case of lexicographic product graphs. Finally, we give tight bounds on the (I)MV chromatic numbers for the Cartesian and strong product graphs, which lead to exact values in some important families of product graphs.
Keywords: independent mutual visibility, mutual-visibility coloring, independence number, Ramsey number, graph product, diameter 2 graph, geodesic
Published in DiRROS: 05.05.2026; Views: 133; Downloads: 91
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Abstract: In this paper, it has been shown that the generalized Petersen graph GP(8, 3), also known as the Möbius-Kantor graph, admits a faithful unit-distance representation in the plane.
Keywords: polycirculant, faithful unit-distance graph, Möbius-Kantor graph, generalized Petersen graph
Published in DiRROS: 27.03.2026; Views: 149; Downloads: 96
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Abstract: Given a connected graph $G$, a set of vertices $X\subset V(G)$ is a weak $k$-resolving set of $G$ if for each two vertices $y,z\in V(G)$, the sum of the values $|d_G(y,x)-d_G(z,x)|$ over all $x\in X$ is at least $k$, where $d_G(u,v)$ stands for the length of a shortest path between $u$ and $v$. The cardinality of a smallest weak $k$-resolving set of $G$ is the weak $k$-metric dimension of $G$, and is denoted by $\mathrm{wdim}_k(G)$. In this paper, $\mathrm{wdim}_k(K_n\,\square\,K_n)$ is determined for every $n\ge 3$ and every $2\le k\le 2n$. An improvement of a known integer linear programming formulation for this problem is developed and implemented for the graphs $K_n\,\square\,K_m$. Conjectures regarding these general situations are posed.
Keywords: weak $k$-metric dimension, weak $k$-resolving set, Cartesian product, Hamming graph
Published in DiRROS: 13.03.2026; Views: 206; Downloads: 151
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Abstract: A nut graph is a simple graph whose kernel is spanned by a single full vector (i.e., the adjacency matrix has a single zero eigenvalue and all non-zero kernel eigenvectors have no zero entry). We classify generalisations of nut graphs to nut digraphs: a digraph whose kernel (resp. co-kernel) is spanned by a full vector is dextro-nut (resp. laevo-nut); a bi-nut digraph is both laevo- and dextro-nut; an ambi-nut digraph is a bi-nut digraph where kernel and co-kernel are spanned by the same vector; a digraph is inter-nut if the intersection of the kernel and co-kernel is spanned by a full vector. It is known that a nut graph is connected, leafless and non-bipartite. It is shown here that an ambi-nut digraph is strongly connected, non-bipartite (i.e., has a non-bipartite underlying graph) and has minimum in-degree and minimum out-degree of at least 2. Refined notions of core and core-forbidden vertices apply to singular digraphs. Infinite families of nut digraphs and systematic coalescence, crossover and multiplier constructions are introduced. Relevance of nut digraphs to topological physics is discussed.
Keywords: nut graph, core graph, nullity, directed graph, nut digraph, dextro-nut, laevo-nut, bi-nut, ambi-nut, inter-nut, dextro-core vertex, laevo-core vertex, graph spectra
Published in DiRROS: 09.03.2026; Views: 236; Downloads: 143
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