An Algebraic Formulation of the Graph Reconstruction Conjecture

The graph reconstruction conjecture asserts that every finite simple graph on at least three vertices can be reconstructed up to isomorphism from its deck—the collection of its vertex‐deleted subgraphs. Kocay's Lemma is an important tool in graph reconstruction. Roughly speaking, given the deck of a graph G and any finite sequence of graphs, it gives a linear constraint that every reconstruction of G must satisfy. Let ψ ( n ) be the number of distinct (mutually nonisomorphic) graphs on n vertices, and let d ( n ) be the number of distinct decks that can be constructed from these graphs. Then the difference ψ ( n ) − d ( n ) measures how many graphs cannot be reconstructed from their decks. In particular, the graph reconstruction conjecture is true for n ‐vertex graphs if and only if ψ ( n ) = d ( n ) . We give a framework based on Kocay's lemma to study this discrepancy. We prove that if M is a matrix of covering numbers of graphs by sequences of graphs, then d ( n ) ≥ rank R ( M ) . In particular, all n ‐vertex graphs are reconstructible if one such matrix has rank ψ ( n ) . To complement this result, we prove that it is possible to choose a family of sequences of graphs such that the corresponding matrix M of covering numbers satisfies d ( n ) = rank R ( M ) .