Mathematics – Combinatorics
Scientific paper
2006-05-28
Mathematics
Combinatorics
46 pages, 3 figures corrections, additions and improved discussions of references. Editorial and spelling corrections; more ac
Scientific paper
The Tutte equations are ported (or set-pointed) when the equations F(N) = g_e F(N/e) + r_e F(N\e) are omitted for elements e in a distinguished set called ports. Solutions F can distinguish different orientations of the same matroid. A ported extensor with ground set is a decomposible element in the exterior algebra over a vector space with a given basis, called the ground set, containing a distinguished subset called ports. These can represent ported matroids and have analogous dualization, deletion and contraction operations. A ported extensor function is defined using dualization, port element renaming, exterior multiplication, and contraction of non-ports. We prove that this function satisfies a sign-corrected variant of the Tutte equations over exterior algebra. For non-ported unimodular, i.e., regular matroids, our function reduces to the basis generating function and for graphs the Laplacian (or Kirchhoff) determinant. In general, the function value, as an extensor, signifies the space of solutions to Kirchhoff's and Ohm's electricity equations after projection to the variables associated to the ports. Combinatorial interpretation of various determinants (the Plucker coordinates) generalize the matrix tree theorem and forest enumeration expressions for electrical resistance. The corank-nullity polynomial, basis expansions with activities, and a geometric lattice expansion generalize to ported Tutte functions of oriented matroids. The ported Tutte functions are parametrized, which raises the problem of how to generalize known characterizations of parameterized non-ported Tutte functions.
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