A picture P of a graph G = (V,E) consists of a point P(v) for each vertex v
in V and a line P(e) for each edge e in E, all lying in the projective plane
over a field k and subject to containment conditions corresponding to incidence
in G. A graph variety is an algebraic set whose points parametrize pictures of
G. We consider three kinds of graph varieties: the picture space X(G) of all
pictures, the picture variety V(G), an irreducible component of X(G) of
dimension 2|V|, defined as the closure of the set of pictures on which all the
P(v) are distinct, and the slope variety S(G), obtained by forgetting all data
except the slopes of the lines P(e). We use combinatorial techniques (in
particular, the theory of combinatorial rigidity) to obtain the following
geometric and algebraic information on these varieties: (1) a description and
combinatorial interpretation of equations defining each variety
set-theoretically; (2) a description of the irreducible components of X(G); and
(3) a proof that V(G) and S(G) are Cohen-Macaulay when G satisfies a sparsity
condition, rigidity independence. In addition, our techniques yield a new proof
of the equality of two matroids studied in rigidity theory.Comment: 19 pages. To be published in Transactions of the AM
