Let G be a connected graph and ℓ → E G : ( ) + a length-function on the edges of G. The Steiner distance sd G (A) of A ⊆ V(G) within G is the minimum length of a connected subgraph of G containing A,where the length of a subgraph is the sum of the lengths of its edges. It is clear that every subgraph. It is easy to construct examples of graphs H ⊆ G such that H is k-isometric, but not (k + 1)-isometric, so this defines a strict hierarchy of properties. We are interested in situations in which this hierarchy collapses in the sense that if H ⊆ G is k-isometric, then H is already fully isometric in G. Our first result of this kind asserts that if T is a tree and T ⊆ G is 2-isometric with respect to some lengthfunction ℓ, then it is fully isometric. This fails for graphs containing a cycle. We then prove that if C is a cycle and C ⊆ G is 6-isometric, then C is fully isometric. We present an example showing that the number 6 is indeed optimal. We then develop a structural approach toward a more general theory and present several open questions concerning the big picture underlying this phenomenon.
K E Y W O R D Sgraph theory, isometry, steiner distance, steiner tree How to cite this article: Weißauer D. Isometric subgraphs for Steiner distance. J Graph Theory. 2020;94:597-613. https://doi.