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Editorial Team
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Asked: 2026-05-22T23:24:33+00:00

g1=Graph[{UndirectedEdge[a,b]}]; GraphQ[g1] (* OUT: True *) (*Needs[Combinatorica`]*) PlanarQ[g1] (* OUT: PlanarQ[.-.] *) Combinatorica`PlanarQ[g1] (*

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 g1=Graph[{UndirectedEdge[a,b]}];
 GraphQ[g1]
 (*
   OUT: True
 *)
   (*Needs["Combinatorica`"]*)
 PlanarQ[g1]
 (*
    OUT: PlanarQ[.-.]
 *)
 Combinatorica`PlanarQ[g1]
 (*
   OUT: Combinatorica`PlanarQ[.-.]
 *)

Why doesn’t PlanarQ give back “True” or “False” ?

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1 Answer

  1. Editorial Team

    Your graph is not a Combinatorica graph, rather it is a System graph. You’ll need to specify the contexts explicitly.

    Needs["GraphUtilities`"];
g1 = System`Graph[{UndirectedEdge[a, b]}];
Combinatorica`PlanarQ[
  GraphUtilities`ToCombinatoricaGraph[g1]]

    This returns True but, in general, this process is a pain and rather buggy. I believe that Combinatorica is on the way out and I’d recommend trying to get by without.

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