Wikipedia:Reference desk/Archives/Mathematics/2019 September 26

= September 26 =

Is there a standard textual method of describing a planar graph?
I was pondering some planar graphs and wanted to list/index them, so I began to consider how this might be done, but then realized that it is highly likely that there exists some standard already.

FWIW, My embryonic method listed each node with the number of edges reaching it and then listing each chain from it in parentheses, repeating as required. I am aware that this has many flaws and doesn't work, but it is as far as I got before considering what may already be out there. My method comes unstuck with loops! (A 4 node snake might be "2(21)(1)" sorting the nodes in descending order of edges - The same snake might be "1221" if you don't care about that.)

Anyway - is there a scheme detailed anywhere? -- SGBailey (talk) 21:56, 26 September 2019 (UTC)


 * How about if you describe the links of the planar graph rather than the nodes ? That is, you have a table with each link as a single row, with the start and end node as the entries for each link/row. So, logically more like a relational database than what you had in mind, which was more like a hierarchical database. SinisterLefty (talk) 22:03, 26 September 2019 (UTC)


 * Yes, thanks. That would work and would cope with loops. In that scheme my 4 node (1,2,3,4) snake with three edges might be "(1,2)(2,3)(3,4)". However is there any industry standard already in use? -- SGBailey (talk) 22:09, 26 September 2019 (UTC)


 * There is a Python library for graphs, see . There doesn't see to be any functions included that create a graph from a text file in any standardized way, as you'd think there would be if there was an industry standard; apparently it just uses lists of edges. I don't see how knowing the graph is planar would help with organizing the data. If anything, to make use of planarity you'd need to store the dual as well as the original and that would double the amount of storage. --RDBury (talk) 22:17, 27 September 2019 (UTC)


 * Yes, I suggest actually storing the images of the planar graphs in a way that users can select the graph from the image (use thumbnails, then click on one to see the full zoom-able and pan-able image). Trying to describe such a planar graph in a non-visual way that a human can understand just doesn't seem practical, beyond the tiniest of graphs. SinisterLefty (talk) 22:28, 27 September 2019 (UTC)
 * There is the Graphviz language. It's designed for visualisation, so it comes with a lot of extra features. But you can just use a list of links (and a lean wrapper), and have the extra bonus of getting a free image. --Stephan Schulz (talk) 21:23, 28 September 2019 (UTC)