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We say that a graph or a multigraph is locally irregular if adjacent vertices have different degrees. The locally irregular edge coloring is an edge coloring of a (multi)graph $G$ such that every color induces a locally irregular sub(multi)graph of $G$. We denote by ${\rm lir}(G)$ the locally irregular chromatic index of $G$, which is the smallest number of colors required in a locally irregular edge coloring of $G$ if such a coloring exists. We state the following new problem. Let $G$ be a connected graph that is not isomorphic to $K_2$ or $K_3$. What is the minimum number of edges of $G$ whose doubling yields a multigraph which is locally irregular edge colorable using at most two colors with no multiedges colored with two colors? This problem is closely related to the Local Irregularity Conjecture for graphs, (2, 2)-Conjecture, Local Irregularity Conjecture for 2-multigraphs and other similar concepts concerning edge colorings. We solve this problem for graph classes like paths, cycles, trees, complete graphs, complete $k$-partite graphs, split graphs and powers of cycles. Our solution of this problem for complete $k$-partite graphs ($k>1$) and powers of cycles which are not complete graphs shows that the locally irregular chromatic index is equal to two for these graph classes. We also consider this problem for some special families of cacti and prove that the minimum number of edges in a graph whose doubling yields a multigraph which has such coloring does not have a constant upper bound not only for locally irregular uncolorable cacti., Comment: 36 pages, 8 figures

For $D$ being a subset of positive integers, the integer distance graph is the graph $G(D)$, whose vertex set is the set of integers, and edge set is the set of all pairs $uv$ with $|u-v| \in D$. It is known that $\chi(G(D)) \leq |D|+1$. This article studies the problem (which is motivated by a conjecture of Zhu): "Is it true that $\chi(G(D)) = |D|+1$ implies $\omega(G(D)) \geq |D|+1$, where $\omega(H)$ is the clique number of $H$?". We give a negative answer to this question, by showing an infinite class of integer distance graphs with $\chi(G(D))=|D|+1$ but $\omega(G(D))=|D|-1$.

A graph is {\em locally irregular} if no two adjacent vertices have the same degree. A {\em locally irregular edge-coloring} of a graph $G$ is such an (improper) edge-coloring that the edges of any fixed color induce a locally irregular graph. Among the graphs admitting a locally irregular edge-coloring, i.e., {\em decomposable graphs}, only one is known to require $4$ colors, while for all the others it is believed that $3$ colors suffice. In this paper, we prove that decomposable claw-free graphs with maximum degree $3$, all cycle permutation graphs, and all generalized Petersen graphs admit a locally irregular edge-coloring with at most $3$ colors. We also discuss when $2$ colors suffice for a locally irregular edge-coloring of cubic graphs and present an infinite family of cubic graphs of girth $4$ which require $3$ colors.

A matching $M$ in a graph $G$ is {\em semistrong} if every edge of $M$ has an endvertex of degree one in the subgraph induced by the vertices of $M$. A {\em semistrong edge-coloring} of a graph $G$ is a proper edge-coloring in which every color class induces a semistrong matching. In this paper, we continue investigation of properties of semistrong edge-colorings initiated by Gy\'{a}rf\'{a}s and Hubenko ({Semistrong edge coloring of graphs}. \newblock {\em J. Graph Theory}, 49 (2005), 39--47). We establish tight upper bounds for general graphs and for graphs with maximum degree $3$. We also present bounds about semistrong edge-coloring which follow from results regarding other, at first sight non-related, problems. We conclude the paper with several open problems.

A {\em conflict-free coloring} of a graph {\em with respect to open} (resp., {\em closed}) {\em neighborhood} is a coloring of vertices such that for every vertex there is a color appearing exactly once in its open (resp., closed) neighborhood. Similarly, a {\em unique-maximum coloring} of a graph {\em with respect to open} (resp., {\em closed}) {\em neighborhood} is a coloring of vertices such that for every vertex the maximum color appearing in its open (resp., closed) neighborhood appears exactly once. There is a vast amount of literature on both notions where the colorings need not be proper, i.e., adjacent vertices are allowed to have the same color. In this paper, we initiate a study of both colorings in the proper settings with the focus given mainly to planar graphs. We establish upper bounds for the number of colors in the class of planar graphs for all considered colorings and provide constructions of planar graphs attaining relatively high values of the corresponding chromatic numbers. As a main result, we prove that every planar graph admits a proper unique-maximum coloring with respect to open neighborhood with at most 10 colors, and give examples of planar graphs needing at least $6$ colors for such a coloring. We also establish tight upper bounds for outerplanar graphs. Finally, we provide several new bounds also for the improper setting of considered colorings.

A proper edge coloring of a graph is strong if it creates no bichromatic path of length three. It is well known that for a strong edge coloring of a $k$-regular graph at least $2k-1$ colors are needed. We show that a $k$-regular graph admits a strong edge coloring with $2k-1$ colors if and only if it covers the Kneser graph $K(2k-1,k-1)$. In particular, a cubic graph is strongly $5$-edge-colorable whenever it covers the Petersen graph. One of the implications of this result is that a conjecture about strong edge colorings of subcubic graphs proposed by Faudree et al. [Ars Combin. 29 B (1990), 205--211] is false.

A cyclic coloring of a plane graph $G$ is a coloring of its vertices such that vertices incident with the same face have distinct colors. The minimum number of colors in a cyclic coloring of a plane graph $G$ is its cyclic chromatic number $\chi_c(G)$. Let $\Delta^*(G)$ be the maximum face degree of a graph $G$. In this note we show that to prove the Cyclic Coloring Conjecture of Borodin from 1984, saying that every connected plane graph $G$ has $\chi_c(G) \leq \lfloor \frac{3}{2}\Delta^*(G)\rfloor$, it is enough to do it for subdivisions of simple $3$-connected plane graphs. We have discovered four new different upper bounds on $\chi_c(G)$ for graphs $G$ from this restricted family; three bounds of them are tight. As corollaries, we have shown that the conjecture holds for subdivisions of plane triangulations, simple $3$-connected plane quadrangulations, and simple $3$-connected plane pentagulations with an even maximum face degree, for regular subdivisions of simple $3$-connected plane graphs of maximum degree at least 10, and for subdivisions of simple $3$-connected plane graphs having the maximum face degree large enough in comparison with the number of vertices of their longest paths consisting only of vertices of degree two.

A star edge-coloring of a graph $G$ is a proper edge-coloring without bichromatic paths or cycles of length four. The smallest integer $k$ such that $G$ admits a star edge-coloring with $k$ colors is the star chromatic index of $G$. In the seminal paper on the topic, Dvo\v{r}\'{a}k, Mohar, and \v{S}\'{a}mal asked if the star chromatic index of complete graphs is linear in the number of vertices and gave an almost linear upper bound. Their question remains open, and consequently, to better understand the behavior of the star chromatic index, this parameter has been studied for a number of other classes of graphs. In this paper, we consider star edge-colorings of square grids; namely, the Cartesian products of paths and cycles and the Cartesian products of two cycles. We improve previously established bounds and, as a main contribution, we prove that the star chromatic index of graphs in both classes is either $6$ or $7$ except for prisms. Additionally, we give a number of exact values for many considered graphs.

A graph is $1$-planar if it has a drawing in the plane such that each edge is crossed at most once by another edge. Moreover, if this drawing has the additional property that for each crossing of two edges the end vertices of these edges induce a complete subgraph, then the graph is locally maximal $1$-planar. For a $3$-connected locally maximal $1$-planar graph $G$, we show the existence of a spanning $3$-connected planar subgraph and prove that $G$ is hamiltonian if $G$ has at most three $3$-vertex-cuts, and that $G$ is traceable if $G$ has at most four $3$-vertex-cuts. Moreover, infinitely many non-traceable $5$-connected $1$-planar graphs are presented.

A $d$-subsequence of a sequence $\varphi = x_1\dots x_n$ is a subsequence $x_i x_{i+d} x_{i+2d} \dots$, for any positive integer $d$ and any $i$, $1 \le i \le n$. A \textit{$k$-Thue sequence} is a sequence in which every $d$-subsequence, for $1 \le d \le k$, is non-repetitive, i.e. it contains no consecutive equal subsequences. In 2002, Grytczuk proposed a conjecture that for any $k$, $k+2$ symbols are enough to construct a $k$-Thue sequences of arbitrary lengths. So far, the conjecture has been confirmed for $k \in \{1,2,3,5\}$. Here, we present two different proving techniques, and confirm it for all $k$, with $2 \le k \le 8$.

Deciding whether a planar graph (even of maximum degree $4$) is $3$-colorable is NP-complete. Determining subclasses of planar graphs being $3$-colorable has a long history, but since Gr\"{o}tzsch's result that triangle-free planar graphs are such, most of the effort was focused to solving Havel's and Steinberg's conjectures. In this paper, we prove that every planar graph of maximum degree $4$ obtained as a subgraph of the medial graph of any bipartite plane graph is $3$-choosable. These graphs are allowed to have close triangles (even incident), and have no short cycles forbidden, hence representing an entirely different class than the graphs inferred by the above mentioned conjectures.

{\emph A star edge-coloring} of a graph is a proper edge-coloring without bichromatic paths and cycles of length four. In this paper, we consider the list version of this coloring and prove that the list star chromatic index of every subcubic graph is at most $7$, answering the question of Dvo\v{r}\'{a}k et al. (Star chromatic index, J. Graph Theory 72 (2013), 313--326).

A graph is \textit{locally irregular} if the neighbors of every vertex $v$ have degrees distinct from the degree of $v$. \textit{locally irregular edge-coloring} of a graph $G$ is an (improper) edge-coloring such that the graph induced on the edges of any color class is locally irregular. It is conjectured that $3$ colors suffice for a locally irregular edge-coloring. Recently, Bensmail et al. (Bensmail, Merker, Thomassen: Decomposing graphs into a constant number of locally irregular subgraphs, {\em European J. Combin.}, 60:124--134, 2017) settled the first constant upper bound for the problem to $328$ colors. In this paper, using a combination of existing results, we present an improvement of the bounds for bipartite graphs and general graphs, setting the best upper bounds to $7$ and $220$, respectively. In addition, we also prove that $4$ colors suffice for locally irregular edge-coloring of any subcubic graph.

An incidence in a graph $G$ is a pair $(v,e)$ where $v$ is a vertex of $G$ and $e$ is an edge of $G$ incident to $v$. Two incidences $(v,e)$ and $(u,f)$ are adjacent if at least one of the following holds: $(a)$ $v = u$, $(b)$ $e = f$, or $(c)$ $vu \in \{e,f\}$. An incidence coloring of $G$ is a coloring of its incidences assigning distinct colors to adjacent incidences. It was conjectured that at most $\Delta(G) + 2$ colors are needed for an incidence coloring of any graph $G$. The conjecture is false in general, but the bound holds for many classes of graphs. We introduce some sufficient properties of the two factor graphs of a Cartesian product graph $G$ for which $G$ admits an incidence coloring with at most $\Delta(G) + 2$ colors., Comment: 11 pages, 5 figures

A strong edge coloring of a graph $G$ is a proper edge coloring in which each color class is an induced matching of $G$. In 1993, Brualdi and Quinn Massey proposed a conjecture that every bipartite graph without $4$-cycles and with the maximum degrees of the two partite sets $2$ and $\Delta$ admits a strong edge coloring with at most $\Delta+2$ colors. We prove that this conjecture holds for such graphs with $\Delta=3$. We also confirm the conjecture proposed by Faudree et al. for subcubic bipartite graphs.

\textit{A star edge coloring} of a graph is a proper edge coloring without bichromatic paths and cycles of length four. In this paper we establish tight upper bounds for trees and subcubic outerplanar graphs, and derive an upper bound for outerplanar graphs.

A strong edge coloring of a graph is a proper edge coloring where the edges at distance at most two receive distinct colors. It is known that every planar graph with maximum degree D has a strong edge coloring with at most 4D + 4 colors. We show that 3D + 6 colors suffice if the graph has girth 6, and 3D colors suffice if the girth is at least 7. Moreover, we show that cubic planar graphs with girth at least 6 can be strongly edge colored with at most 9 colors.

An l-facial edge coloring of a plane graph is a coloring of the edges such that any two edges at distance at most l on a boundary walk of some face receive distinct colors. It is conjectured that 3l + 1 colors suffice for an l-facial edge coloring of any plane graph. We prove that 7 colors suffice for a 2-facial edge coloring of any plane graph and therefore confirm the conjecture for l = 2.

For a given graph H and n ? 1; let f(n;H) denote the maximum number m for which it is possible to colour the edges of the complete graph Kn with m colours in such a way that each subgraph H in Kn has at least two edges of the same colour. Equivalently, any edge-colouring of Kn with at least rb(n;H) = f(n;H)+1 colours contains a rainbow copy of H: The numbers f(n;H) and rb(Kn;H) are called anti-ramsey numbers and rainbow numbers, respectively. In this paper we will classify the rainbow number for a given graph H with respect to its cyclomatic number. Let H be a graph of order p >= 4 and cyclomatic number v(H) >= 2: Then rb(Kn;H) cannot be bounded from above by a function which is linear in n: If H has cyclomatic number v(H) = 1; then rb(Kn;H) is linear in n: We will compute all rainbow numbers for the bull B; which is the unique graph with 5 vertices and degree sequence (1; 1; 2; 3; 3): Furthermore, we will compute some rainbow numbers for the diamond D = K_4 - e; for K_2;3; and for the house H (complement of P_5).