The R graph is the graph on 6 vertices illustrated above.
It is implemented in the Wolfram Language as GraphData["RGraph"].
See also
A Graph, Banner Graph, E Graph, Fish Graph, H Graph
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References
Brandstädt, A.; Le, V. B.; and Spinrad, J. P. Graph Classes: A Survey. Philadelphia, PA: SIAM, p. 19, 1987.ISGCI: Information System on Graph Class Inclusions v2.0. "List of Small Graphs." http://www.graphclasses.org/smallgraphs.html.
Referenced on Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "R Graph." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/RGraph.html