planar graph G
Pfaffian method
Pfaffian orientation
Let A denote the signed
adjacency matrix of G. This means that
• Aij = 0 if no edge joins vi to vj .
• Aij = 1 if the edge joining vi to vj is oriented from vi to vj .
• Aij = −1 if the edge joining vi to vj is oriented from vj to vi .
A perfect matching of G is a collection of edges e1, ..., en of G such that every vertex belongs to exactly one ej .
Let P(G) denote the number of distinct perfect matchings of G. Then
The matrix A is called skew-symmetric if Aij = −Aji for all i, j. A is skew-symmetric.
Let A be a 2n × 2n skew-symmetric matrix. The Pfaffian of A is defined as the sum taken over all permutations of the set {1, ..., 2n}.
Given Muir’s Identity, Kastelyn’s Theorem can be reformulated as follows.
Let G be a graph equipped with any Pfaffian orientation. Let A be the corresponding signed adjacency matrix. Then
Muir’s Identity
1
1 Kastelyn’s Formula for Perfect Matchings. Rich Schwartz
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