Planar graphs without mutually adjacent 3-, 5-, and 6-cycles are 3-degenerate

A graph G is k-degenerate if every subgraph of G has a vertex with degree at most k. Using the Euler's formula, one can obtain that planar graphs without 3-cycles are 3-degenerate. Wang and Lih, and Fijav? et al. proved the analogue results for planar graphs without 5-cycles and planar graphs without 6-cycles, respectively. Recently, Liu et al. showed that planar graphs without 3-cycles adjacent to 5-cycles are 3-degenerate. In this work, we generalized all aforementioned results by showing that planar graphs without mutually adjacent 3-,5-, and 6-cycles are 3-degenerate. A graph G without mutually adjacent 3-,5-, and 6-cycles means that G cannot contain three graphs, say $G_1,G_2$, and $G_3$, where $G_1$ is a 3-cycle, $G_2$ is a 5-cycle, and $G_3$ is a 6-cycle such that each pair of $G_1,G_2$, and $G_3$ are adjacent. As an immediate consequence, we have that every planar graph without mutually adjacent 3-,5-, and 6-cycles is DP-4-colorable. This consequence also generalizes the result by Chen et al that planar graphs without 5-cycles adjacent to 6-cycles are DP-4-colorable.     

Adjacent vertex distinguishing total coloring of planar graphs with maximum degree 9

Let $k$ be a positive integer. An adjacent vertex distinguishing (for short, AVD) total-$k$-coloring of a graph $G$ is a proper total-$k$-coloring of $G$ such that any two adjacent vertices have different color sets, where the color set of a vertex $v$ contains the color of $v$ and the colors of its incident edges. It was conjectured that any graph with maximum degree $\Delta$ has an AVD total-($\Delta +3$)-coloring. The conjecture was confirmed for any graph with maximum degree at most 4 and any planar graph with maximum degree at least 10. In this paper, we verify the conjecture for all planar graphs with maximum degree at least 9. Moreover, we prove that any planar graph with maximum degree at least 10 has an AVD total-($\Delta +2$)-coloring and the bound $\Delta +2$ is sharp.     

Every signed planar graph without cycles of length from 4 to 8 is 3-colorable

In this paper, we investigate the signed graph version of Erdös problem: Is there a constant $c$ such that every signed planar graph without $k$-cycles, where $4\le k\le c$, is 3-colorable and prove that each signed planar graph without cycles of length from 4 to 8 is 3-colorable.     

On incidence choosability of cubic graphs

An incidence of a graph $G$ is a pair $(u,e)$ where $u$ is a vertex of $G$ and $e$ is an edge of $G$ incident to $u$. Two incidences $(u,e)$ and $(v,f)$ of $G$ are adjacent whenever (i) $u=v$, or (ii) $e=f$, or (iii) $uv=e$ or $uv=f$. An incidence$k$-coloring of $G$ is a mapping from the set of incidences of $G$ to a set of $k$ colors such that every two adjacent incidences receive distinct colors. The notion of incidence coloring has been introduced by Brualdi and Quinn Massey (1993) from a relation to strong edge coloring, and since then, has attracted a lot of attention by many authors.On a list version of incidence coloring, it was shown by Benmedjdoub et al. (2017) that every Hamiltonian cubic graph is incidence 6-choosable. In this paper, we show that every cubic (loopless) multigraph is incidence 6-choosable. As a direct consequence, it implies that the list strong chromatic index of a $(2,3)$-bipartite graph is at most 6, where a (2,3)-bipartite graph is a bipartite graph such that one partite set has maximum degree at most 2 and the other partite set has maximum degree at most 3.     

A constructive characterization of contraction critical 8-connected graphs with minimum degree 9

An edge of a $k$-connected graph is said to be $k$-contractible if its contraction results in a $k$-connected graph. A $k$-connected graph without $k$-contractible edge is said to be contraction critically $k$-connected. Y. Egawa and W. Mader, independently, showed that the minimum degree of a contraction critical $k$-connected graph is at most $\frac{5k}{4}?1$. Hence, the minimum degree of a contraction critical 8-connected graph is either 8 or 9. This paper shows that a graph $G$ is a contraction critical 8-connected graph with minimum degree 9 if and only if $G$ is the strong product of a contraction critical 4-connected graph $H$ and $K_2$.     

List star edge coloring of k-degenerate graphs

A star edge coloring of a graph is a proper edge coloring such that every connected 2-colored subgraph is a path with at most 3 edges. Deng et al. and Bezegová et al. independently show that the star chromatic index of a tree with maximum degree $\Delta$ is at most $?\frac{3\Delta }{2}?$, which is tight. In this paper, we study the list star edge coloring of $k$-degenerate graphs. Let $c{h}_{st}^{\prime }\left(G\right)$ be the list star chromatic index of $G$: the minimum $s$ such that for every $s$-list assignment $L$ for the edges, $G$ has a star edge coloring from $L$. By introducing a stronger coloring, we show with a very concise proof that the upper bound on the star chromatic index of trees also holds for list star chromatic index of trees, i.e. $c{h}_{st}^{\prime }\left(T\right)\le ?\frac{3\Delta }{2}?$ for any tree $T$ with maximum degree $\Delta$. And then by applying some orientation technique we present two upper bounds for list star chromatic index of $k$-degenerate graphs.     

The first order theory of G(n,c∕n)

A well-known result from the 1960 paper of Erdős and Rényi (1960) [2] tells us that the almost sure theory for first order language on the random graph sequence $\left\{G(n,c{n}^{-1})\right\}$ is not complete. Our paper proposes and proves what the complete set of completions of the almost sure theory for $\left\{G(n,c{n}^{-1})\right\}$ should be. The almost sure theory $T$ consists of two sentence groups: the first states that all the components are trees or unicyclic components, and the second states that, given any $k\in \mathbb{N}$ and any finite tree $t$, there are at least $k$ components isomorphic to $t$. We define a $k$-completion of $T$ to be a first order property $A$, such that if $T\cup A$ holds for a graph (which indicates that the property described in sentence $A$ is satisfied by the graph, and for every sentence $B$ in the theory $T$, the property described by $B$ is also satisfied by the graph), we can fully describe the first order sentences of quantifier depth $\le k$ that hold for that graph. We show that a $k$-completion $A$ specifies the numbers, up to “cutoff” $k$, of the (finitely many) unicyclic component types of given parameters (that only depend on $k$) that the graph contains. A complete set of $k$-completions is then the finite collection of all possible $k$-completions.     

Long paths and toughness of k-trees and chordal planar graphs

We show that every $k$-tree of toughness greater than $\frac{k}{3}$ is Hamilton-connected for $k\ge 3$. (In particular, chordal planar graphs of toughness greater than 1 are Hamilton-connected.) This improves the result of Broersma et al. (2007) and generalizes the result of Böhme et al. (1999).On the other hand, we present graphs whose longest paths are short. Namely, we construct 1-tough chordal planar graphs and 1-tough planar 3-trees, and we show that the shortness exponent of the class is 0, at most ${log}_{30}22$, respectively. Both improve the bound of Böhme et al. Furthermore, the construction provides $k$-trees (for $k\ge 4$) of toughness greater than 1.     

Improved square coloring of planar graphs

Square coloring is a variant of graph coloring where vertices within distance two must receive different colors. When considering planar graphs, the most famous conjecture (Wegner, 1977) states that $\frac{3}{2}\mathrm{\Delta }+1$ colors are sufficient to square color every planar graph of maximum degree Δ. This conjecture has been proven asymptotically for graphs with large maximum degree. We consider here planar graphs with small maximum degree and show that $2\mathrm{\Delta }+7$ colors are sufficient, which improves the best known bounds when $6?\mathrm{\Delta }?31$.     

H-free subgraphs of dense graphs maximizing the number of cliques and their blow-ups

We consider the structure of $H$-free subgraphs of graphs with high minimal degree. We prove that for every $k>m$ there exists an $???(k,m)>0$ so that the following holds. For every graph $H$ with chromatic number $k$ from which one can delete an edge and reduce the chromatic number, and for every graph $G$ on $n>n_0\left(H\right)$ vertices in which all degrees are at least $(1??)n$, any subgraph of $G$ which is $H$-free and contains the maximum number of copies of the complete graph $K_m$ is $(k?1)$-colorable.We also consider several extensions for the case of a general forbidden graph $H$ of a given chromatic number, and for subgraphs maximizing the number of copies of balanced blowups of complete graphs.     

Proportional choosability: A new list analogue of equitable coloring

In 2003, Kostochka, Pelsmajer, and West introduced a list analogue of equitable coloring called equitable choosability. In this paper, we motivate and define a new list analogue of equitable coloring called proportional choosability. A $k$-assignment $L$ for a graph $G$ specifies a list $L\left(v\right)$ of $k$ available colors for each vertex $v$ of $G$. An $L$-coloring assigns a color to each vertex $v$ from its list $L\left(v\right)$. For each color $c$, let $\eta \left(c\right)$ be the number of vertices $v$ whose list $L\left(v\right)$ contains $c$. A proportional$L$-coloring of $G$ is a proper $L$-coloring in which each color $c\in {?}_{v\in V\left(G\right)}L\left(v\right)$ is used $?\eta \left(c\right)∕k?$ or $?\eta \left(c\right)∕k?$ times. A graph $G$ is proportionally$k$-choosable if a proportional $L$-coloring of $G$ exists whenever $L$ is a $k$-assignment for $G$. We show that if a graph $G$ is proportionally $k$-choosable, then every subgraph of $G$ is also proportionally $k$-choosable and also $G$ is proportionally $(k+1)$-choosable, unlike equitable choosability for which analogous claims would be false. We also show that any graph $G$ is proportionally $k$-choosable whenever $k\ge \Delta \left(G\right)+?\left|V\left(G\right)\right|∕2?$, and we use matching theory to completely characterize the proportional choosability of stars and the disjoint union of cliques.     

On the (3,1)-choosability of planar graphs without adjacent cycles of length 5,6,7

A $(k,d)$-list assignment $L$ of a graph $G$ is a mapping that assigns to each vertex $v$ a list $L\left(v\right)$ of at least $k$ colors satisfying $|L\left(x\right)\cap L\left(y\right)|\le d$ for each edge $xy$. A graph $G$ is $(k,d)$-choosable if there exists an $L$-coloring of $G$ for every $(k,d)$-list assignment $L$. This concept is also known as choosability with separation. In this paper, we prove that any planar graph $G$ is $(3,1)$-choosable if any $i$-cycle is not adjacent to a $j$-cycle, where $5\le i\le 6$ and $5\le j\le 7$.     

Maximal ideals of regulous functions are not finitely generated

We prove that every maximal ideal in the ring of k-regulous functions, $k\in \mathbb{N}$, on a smooth real affine algebraic variety of dimension $d\ge 2$ is not finitely generated.     

Balls,bins, and embeddings of partial k-star designs

Define a $k$-star to be the complete bipartite graph $K_{1,k}$. In a 2014 article, Hoffman and Roberts prove that a partial $k$-star decomposition of $K_n$ can be embedded in a $k$-star decomposition of $K_{n+s}$ where $s$ is at most $7k?4$ if $k$ is odd and $8k?4$ if $k$ is even. In our work, we offer a straightforward construction for embedding partial $k$-star designs and lower these bounds to $3k?2$ and $4k?2$, respectively.