Mathematics – Combinatorics
Scientific paper
2009-06-15
Mathematics
Combinatorics
v2 has an expanded section on deletion/contraction for directed graphs, and a more detailed proof of Theorem 2.3. To appear in
Scientific paper
10.1016/j.jcta.2010.04.001
We generalize a theorem of Knuth relating the oriented spanning trees of a directed graph G and its directed line graph LG. The sandpile group is an abelian group associated to a directed graph, whose order is the number of oriented spanning trees rooted at a fixed vertex. In the case when G is regular of degree k, we show that the sandpile group of G is isomorphic to the quotient of the sandpile group of LG by its k-torsion subgroup. As a corollary we compute the sandpile groups of two families of graphs widely studied in computer science, the de Bruijn graphs and Kautz graphs.
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