# Combinatorial data for planar graph

I am trying to get sage working with planar embedded graphs. The answer here was quite helpful, but I have a few more questions.

1. Is there a way to get represention of the embedding into sage? The graph6 format forgets the embedding (see the documentation of plantri).

2. Can I presribe the outer face of my embedding when I plot a Graph in sage using .plot(layout='planar')?

Ideally, for an embedded graph, I would like to obtain something like an oriented outer face and a list of oriented inner faces. For example for the graph below I want to have

inner_faces=[[2,0,3],[0,1,4,5,3],[1,2,4],[2,3,5],[2,5,4]]
outer_face=[0,1,2]


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Helloooooo !!

sage: g = graphs.IcosahedralGraph()
sage: g.is_planar(set_embedding=True)
True
sage: g.get_embedding()
{0: [1, 5, 11, 7, 8],
1: [8, 2, 6, 5, 0],
2: [8, 9, 3, 6, 1],
3: [2, 9, 10, 4, 6],
4: [3, 10, 11, 5, 6],
5: [0, 1, 6, 4, 11],
6: [1, 2, 3, 4, 5],
7: [0, 11, 10, 9, 8],
8: [0, 7, 9, 2, 1],
9: [8, 7, 10, 3, 2],
10: [9, 7, 11, 4, 3],
11: [0, 5, 4, 10, 7]}
sage: g.trace_faces()
[[(10, 11), (11, 7), (7, 10)],
[(6, 4), (4, 3), (3, 6)],
[(5, 6), (6, 1), (1, 5)],
[(2, 8), (8, 1), (1, 2)],
[(9, 8), (8, 2), (2, 9)],
[(8, 0), (0, 1), (1, 8)],
[(3, 2), (2, 6), (6, 3)],
[(0, 7), (7, 11), (11, 0)],
[(2, 1), (1, 6), (6, 2)],
[(8, 9), (9, 7), (7, 8)],
[(4, 10), (10, 3), (3, 4)],
[(5, 4), (4, 6), (6, 5)],
[(11, 4), (4, 5), (5, 11)],
[(10, 4), (4, 11), (11, 10)],
[(9, 3), (3, 10), (10, 9)],
[(7, 0), (0, 8), (8, 7)],
[(11, 5), (5, 0), (0, 11)],
[(10, 7), (7, 9), (9, 10)],
[(2, 3), (3, 9), (9, 2)],
[(5, 1), (1, 0), (0, 5)]]


If you want the faces as list of vertices instead of edges, you can do

sage: map(lambda x:[y[0] for y in x],g.trace_faces())


Aaaaand I have no idea of what layout=planar does,sorry :-)

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( 2014-01-11 05:48:52 -0500 )edit

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Last updated: Jan 11 '14