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Finding All Chordless Simple Cycles In A Graph


I am doing a research on networks which consists of polygons with different number of sides. I am trying to find all simple cycles in a network which are chordless. As an example, consider the following graph:


graph = Graph[
{
1 <-> 2, 2 <-> 3, 3 <-> 4, 4 <-> 5, 5 <-> 6, 1 <-> 6,
3 <-> 7, 7 <-> 8, 8 <-> 9, 9 <-> 10, 10 <-> 11, 11 <-> 3,
4 <-> 12, 12 <-> 13, 13 <-> 11
},
VertexLabels -> "Name"
]


enter image description here


{1,2,3,4,5,6}, {3,4,11,12,13},{3,7,8,9,10,11} are rings and we can extract them:


cycles = FindFundamentalCycles[graph];
rings = Sort @* VertexList @* Graph /@ cycles

enter image description here


But the above solution doesn't always work as it might give non-chordless cycles. Consider the following example:


grapht = Graph[
{

1 <-> 2, 1 <-> 3, 2 <-> 4, 4 <-> 5, 5 <-> 6, 4 <-> 6,
6 <-> 7, 3 <-> 5, 3 <-> 9, 5 <-> 8, 8 <-> 9
},
VertexLabels -> "Name"];

enter image description here


Rings (cycles) are:


cyclest = FindFundamentalCycles[grapht]; 
HighlightGraph[grapht, #] & /@ cyclest


enter image description here


But I need to get {4,5,6} as a ring not {1,2,3,4,5,6} since there is an edge in the latter. Is there any way to filter out only chordless cycles?




W Community crosspost



Answer



First, let us find all cycles in the graph. Then, we will filter out the ones that contain chords; this we can detect by checking if the $n$-vertex induced subgraph is isomorphic to a cycle of length $n$ or not.


Let us use your graph as an example:


g = Graph[{1 <-> 2, 1 <-> 3, 2 <-> 4, 4 <-> 5, 5 <-> 6, 4 <-> 6, 
6 <-> 7, 3 <-> 5, 3 <-> 9, 5 <-> 8, 8 <-> 9},
VertexLabels -> "Name"];


cy = VertexList[Graph[#]] & /@ FindCycle[g, Infinity, All];
Select[cy, IsomorphicGraphQ[CycleGraph[Length[#]], Subgraph[g, #]] &]
(* {{4, 5, 6}, {5, 8, 9, 3}, {1, 2, 4, 5, 3}} *)

HighlightGraph[g, %]

Of course, if you have additional constraints, you can simply modify the call to FindCycle with different parameters to only find cycles of length e.g. of size 5, 6, 7, or 8. To achieve this, just do FindCycle[g, {5, 8}, All] instead.


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