Abstract:We use the Steiner distance to define a convexity in the vertex set of a graph, which has a nice behavior in the well-known class of HHD-free graphs. For this graph class, we prove that any Steiner tree of a vertex set is included into the geodesical convex hull of the set, which extends the well-known fact that the Euclidean convex hull contains at least one Steiner tree for any planar point set. We also characterize the graph class where Steiner convexity becomes a convex geometry, and provide a vertex set t… Show more
“…In a similar way, totally balanced hypergraphs and strongly chordal graphs have been characterized as convex geometries of some particular (hyper)graph convexities [16]. More recently, the so-called Steiner convexity was introduced, and it was shown that precisely 3-fan-free chordal graphs are convex geometries with respect to this convexity [6].…”
a b s t r a c tA walk W between two non-adjacent vertices in a graph G is called tolled if the first vertex of W is among vertices from W adjacent only to the second vertex of W , and the last vertex of W is among vertices from W adjacent only to the second-last vertex of W . In the resulting interval convexity, a set S ⊂ V (G) is toll convex if for any two non-adjacent vertices x, y ∈ S any vertex in a tolled walk between x and y is also in S. The main result of the paper is that a graph is a convex geometry (i.e. satisfies the Minkowski-Krein-Milman property stating that any convex subset is the convex hull of its extreme vertices) with respect to toll convexity if and only if it is an interval graph. Furthermore, some well-known types of invariants are studied with respect to toll convexity, and toll convex sets in three standard graph products are completely described.
“…In a similar way, totally balanced hypergraphs and strongly chordal graphs have been characterized as convex geometries of some particular (hyper)graph convexities [16]. More recently, the so-called Steiner convexity was introduced, and it was shown that precisely 3-fan-free chordal graphs are convex geometries with respect to this convexity [6].…”
a b s t r a c tA walk W between two non-adjacent vertices in a graph G is called tolled if the first vertex of W is among vertices from W adjacent only to the second vertex of W , and the last vertex of W is among vertices from W adjacent only to the second-last vertex of W . In the resulting interval convexity, a set S ⊂ V (G) is toll convex if for any two non-adjacent vertices x, y ∈ S any vertex in a tolled walk between x and y is also in S. The main result of the paper is that a graph is a convex geometry (i.e. satisfies the Minkowski-Krein-Milman property stating that any convex subset is the convex hull of its extreme vertices) with respect to toll convexity if and only if it is an interval graph. Furthermore, some well-known types of invariants are studied with respect to toll convexity, and toll convex sets in three standard graph products are completely described.
“…Further results involving Steiner tree problems related to geodesic convexity in graphs can be found in [15,18,26,28,47,55,96,115,116,136,150,152,155,172,180].…”
Section: Theorem 512 ([119]) Every Edge Steiner Set Of a Connected mentioning
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“…While several convexity notions exist for graphs (e.g., g-convexity [7], m-convexity [5,7], ap-convexity [4], tpconvexity [3], Steiner convexity [2,10]), fewer convexity notions have been defined explicitly for hypergraphs. The first hypergraph convexity that has been introduced is simple-path convexity (sp-convexity, for short) [7], which is a generalization of ap-convexity.…”
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