The problem of determining the smallest number of edges, $h(n;\bar{\kappa}\geq r)$, which guarantees that any graph with $n$ vertices and $h(n;\bar{\kappa}\geq r)$ edges will contain a pair of vertices joined by $r$ internally disjoint paths was posed by Erd\"{o}s and Gallai. Bollob\'{a}s considered the problem of determining the largest number of edges $f(n;\bar{\kappa}\leq \ell)$ for graphs with $n$ vertices and local connectivity at most $\ell$. One can see that $f(n;\bar{\kappa}\leq \ell)= h(n;\bar{\kappa}\geq \ell+1)-1$. These two problems had received a wide attention of many researchers in the last few decades. In the above problems, only pairs of vertices connected by internally disjoint paths are considered. In this paper, we study the number of internally disjoint Steiner trees connecting sets of vertices with cardinality at least 3.

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On Extremal Graphs with at Most Two Internally Disjoint Steiner Trees Connecting Any Three Vertices

The generalized $k$-connectivity $\kappa_k(G)$ of a graph $G$ was introduced by Hager before 1985. As its a natural counterpart, we introduced the concept of generalized edge-connectivity $\lambda_k(G)$, recently. In this paper we summarize the known results on the generalized connectivity and generalized edge-connectivity. After an introductory section, the paper is then divided into nine sections: the generalized (edge-)connectivity of some graph classes, algorithms and computational complexity, sharp bounds of $\kappa_k(G)$ and $\lambda_k(G)$, graphs with large generalized (edge-)connectivity, Nordhaus-Gaddum-type results, graph operations, extremal problems, and some results for random graphs and multigraphs. It also contains some conjectures and open problems for further studies.

A survey on the generalized connectivity of graphs

The maximum local connectivity was first introduced by Bollobas. The problem of determining the maximum number of edges in a graph with $$\overline{\kappa }\le \ell $$?¯≤l has been studied extensively. We consider a generalization of the above concept and problem. For $$S\subseteq V(G)$$S⊆V(G) and $$|S|\ge 2$$|S|?2, the generalized local connectivity$$\kappa (S)$$?(S) is the maximum number of internally disjoint trees connecting $$S$$S in $$G$$G. The parameter $$\overline{\kappa }_k(G)=\max \{\kappa (S)\,|\,S\subseteq V(G),|S|=k\}$$?¯k(G)=max{?(S)|S⊆V(G),|S|=k} is called the maximum generalized local connectivity of $$G$$G. In this paper the problem of determining the largest number $$f(n;\overline{\kappa }_k\le \ell )$$f(n??¯k≤l) of edges for graphs of order $$n$$n that have maximum generalized local connectivity at most $$\ell $$l is considered. The exact value of $$f(n;\overline{\kappa }_k\le \ell )$$f(n??¯k≤l) for $$k=n,n-1$$k=n,n-1 is determined. For a general $$k$$k, we construct a graph to obtain a sharp lower bound.

On Extremal Graphs with at Most $$\ell $$ℓ Internally Disjoint Steiner Trees Connecting Any $$n-1$$n-1 Vertices

A graph G has maximal local edge‐connectivity k if the maximum number of edge‐disjoint paths between every pair of distinct vertices x and y is at most k. We prove Brooks‐type theorems for k‐connected graphs with maximal local edge‐connectivity k, and for any graph with maximal local edge‐connectivity 3. We also consider several related graph classes defined by constraints on connectivity. In particular, we show that there is a polynomial‐time algorithm that, given a 3‐connected graph G with maximal local connectivity 3, outputs an optimal coloring for G. On the other hand, we prove, for urn:x-wiley:03649024:media:jgt22109:jgt22109-math-0001, that k‐colorability is NP‐complete when restricted to minimally k‐connected graphs, and 3‐colorability is NP‐complete when restricted to urn:x-wiley:03649024:media:jgt22109:jgt22109-math-0002‐connected graphs with maximal local connectivity k. Finally, we consider a parameterization of k‐colorability based on the number of vertices of degree at least urn:x-wiley:03649024:media:jgt22109:jgt22109-math-0003, and prove that, even when k is part of the input, the corresponding parameterized problem is FPT.

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Coloring Graphs with Constraints on Connectivity

A spanning connectedness property is one which involves the robust existence of a spanning subgraph which is of some special form, say a Hamiltonian cycle in which a sequence of vertices appear in an arbitrarily given ordering, or a Hamiltonian path in the subgraph obtained by deleting any three vertices, or three internally-vertex-disjoint paths with any given endpoints such that the three paths meet every vertex of the graph and cover the edges of an almost arbitrarily given linear forest of a certain fixed size. Let π = π1 · · · πn be an ordering of the vertices of an n-vertex graph G. For any positive integer k ≤ n − 1, we call π a k-thick Hamiltonian vertex ordering of G provided it holds for all i ∈ {1,. .. , n − 1} that πiπi+1 ∈ E(G) and the number of neighbors of πi among {πi+1,. .. , πn} is at least min{n − i, k}; For any nonnegative integer k, we say that π is a −k-thick Hamiltonian vertex ordering of G provided |{i : πiπi+1 / ∈ E(G), 1 ≤ i ≤ n − 1}| ≤ k + 1. Our main discovery is that the existence of a thick Hamiltonian vertex ordering will guarantee that the graph has various kinds of spanning connectedness properties and that for interval graphs, quite a few seemingly unrelated spanning connectedness properties are equivalent to the existence of a thick Hamiltonian vertex ordering. Due to the connection between Hamiltonian thickness and spanning connectedness properties, we can present several linear time algorithms for associated problems. This paper suggests that much work in graph theory may have a spanning version which deserves further study, and that the Hamiltonian thickness may be a useful concept in understanding many spanning connectedness properties.

https://hal.inria.fr/hal-01337591/document

Bo Aagaard Sørensen

Papers

On k-Rails in Graphs