A Kruskal–Katona type theorem for graphs

AbstractA bound on consecutive clique numbers of graphs is established. This bound is evaluated and shown to often be much better than the bound of the Kruskal–Katona theorem. A bound on non-consecutive clique numbers is also proven

Chromatic Thresholds of Regular Graphs with Small Cliques

The chromatic threshold of a class of graphs is the value θ such that any graph in this class with a minimum degree greater than θn has a bounded chromatic number. Several important results related to the chromatic threshold of triangle-free graphs have been reached in the last 13 years, culminating in a result by Brandt and Thomassé stating that any triangle-free graph on n vertices with minimum degree exceeding 1/3 n has chromatic number at most 4. In this paper, the researcher examines the class of triangle-free graphs that are additionally regular. The researcher finds that any triangle-free graph on n vertices that is regular of degree (1/4+a)n with a \u3e 0 has chromatic number bounded by f (a), a function of a independent of the order of the graph n. After obtaining this result, the researcher generalizes this method to graphs that are free of larger cliques in order to limit the possible values of the chromatic threshold for regular Kr-free graphs

The size-Ramsey number of powers of bounded degree trees

Given a positive integer (Formula presented.), the (Formula presented.) -colour size-Ramsey number of a graph (Formula presented.) is the smallest integer (Formula presented.) such that there exists a graph (Formula presented.) with (Formula presented.) edges with the property that, in any colouring of (Formula presented.) with (Formula presented.) colours, there is a monochromatic copy of (Formula presented.). We prove that, for any positive integers (Formula presented.) and (Formula presented.), the (Formula presented.) -colour size-Ramsey number of the (Formula presented.) th power of any (Formula presented.) -vertex bounded degree tree is linear in (Formula presented.). As a corollary, we obtain that the (Formula presented.) -colour size-Ramsey number of (Formula presented.) -vertex graphs with bounded treewidth and bounded degree is linear in (Formula presented.), which answers a question raised by Kamčev, Liebenau, Wood and Yepremyan

Subgraph densities in a surface

Given a fixed graph $H$ that embeds in a surface $\Sigma$, what is the maximum number of copies of $H$ in an $n$-vertex graph $G$ that embeds in $\Sigma$? We show that the answer is $\Theta(n^{f(H)})$, where $f(H)$ is a graph invariant called the `flap-number' of $H$, which is independent of $\Sigma$. This simultaneously answers two open problems posed by Eppstein (1993). When $H$ is a complete graph we give more precise answers.Comment: v4: referee's comments implemented. v3: proof of the main theorem fully rewritten, fixes a serious error in the previous version found by Kevin Hendre

When Maximum Stable Set Can Be Solved in FPT Time

Maximum Independent Set (MIS for short) is in general graphs the paradigmatic W[1]-hard problem. In stark contrast, polynomial-time algorithms are known when the inputs are restricted to structured graph classes such as, for instance, perfect graphs (which includes bipartite graphs, chordal graphs, co-graphs, etc.) or claw-free graphs. In this paper, we introduce some variants of co-graphs with parameterized noise, that is, graphs that can be made into disjoint unions or complete sums by the removal of a certain number of vertices and the addition/deletion of a certain number of edges per incident vertex, both controlled by the parameter. We give a series of FPT Turing-reductions on these classes and use them to make some progress on the parameterized complexity of MIS in H-free graphs. We show that for every fixed t >=slant 1, MIS is FPT in P(1,t,t,t)-free graphs, where P(1,t,t,t) is the graph obtained by substituting all the vertices of a four-vertex path but one end of the path by cliques of size t. We also provide randomized FPT algorithms in dart-free graphs and in cricket-free graphs. This settles the FPT/W[1]-hard dichotomy for five-vertex graphs H