On the impossibility to construct diametrically critical graphs by extensions
A graph is called an -graph if its periphery is equal to its center eccentric vertices . Further, a graph is called a -graph if . We describe -graphs and -graphs for small radius. Then, for a given graph and natural numbers , , we construct an -graph of radius having central vertices and containing as an induced subgraph. We prove an analogous existence theorem for -graphs, too. At the end, we give some properties of -graphs and -graphs.
The paper gives an overview of results for radially minimal, critical, maximal and stable graphs and digraphs.
We define digraphs minimal, critical, and maximal by three types of radii. Some of these classes are completely characterized, while for the others it is shown that they are large in terms of induced subgraphs.
The known relation between the standard radius and diameter holds for graphs, but not for digraphs. We show that no upper estimation is possible for digraphs. We also give some remarks on distances, which are either metric or non-metric.
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