Homotopy equivalence in graph-like digital topological spaces

For digital topological spaces defined using adjacency relations, there is an established homotopy equivalence relation which parallels that of classical topology. Many classical homotopy equivalence invariants, such as the Euler characteristic and the homology groups, do not remain invariants in the digital setting. This paper develops two numeric digital homotopy invariants and begins to catalog all possible connected digital spaces on a small number of points, up to homotopy equivalence.