6 .Nd finds matroids of plabic graphs
15 graph, is a collection of vertices, undirected edges, and a color
16 associated to each vertex: either black or white. Edges may either be
17 between two vertices, or from some vertex to a point on
19 of the graph: a circle enclosing the entire graph. The edges cannot
21 .Pq this is what it means to be Em planar
22 , and the graph need not be connected. A vertex can only be
23 adjacent to one boundary edge: such vertices are called
26 We may consider turning a plabic graph from an undirected graph into a
27 directed one, simply by assigning directions to each edge. This turns
28 the plabic graph into a
31 A network associated to a plabic graph is
32 .Em perfectly oriented
33 if the following conditions are satisfied.
36 For every white vertex, exactly one edge points
40 For every black vertex, exactly one edge points
45 Most plabic graphs admit many networks. Some (maybe none) of those
46 networks are perfectly oriented. For a number of interesting
47 properties, it turns out that all that matters is the directions of
48 the boundary edges. A boundary vertex whose boundary edge points away
50 .Pq towards the boundary
53 A boundary vertex whose boundary edge points towards it is called a
57 of a plabic graph is the set of collections of sink vertices which
58 admit a perfectly oriented network: i.e. fixing these boundary
59 vertices as sinks, and the other boundary vertices as sources, allows
60 a choice of edge directions such that the resulting plabic network is
61 perfectly oriented. For more information, see Postnikov's paper.
64 .%A Alexander Postnikov
66 .%T Total Positivity, Grassmannians, and Networks
67 .%O Preprint at http://arxiv.org/abs/math/0609764
72 program must read a description of a plabic graph from a file. The
73 file is of the following form:
76 .Dl Ar name Ar name Ar name Ar ...
77 .Dl Ar name Ar name Ar name Ar ...
81 .Dl Ar name Ar name Ar name Ar ...
84 .Dl Po Ar name Ar name Pc Po Ar name Ar name Pc Ar ...
88 is the name of a vertex: a collection of non-whitespace characters
96 Those vertices listed following
98 are colored white, those following
100 are colored black. No duplication is allowed. Edges are listed
103 marker, and must be either between two
105 vertices or between a vertex and the special string
107 which is not allowed as a vertex name. An edge between
113 as a boundary vertex.