Fault diagnosis of multiprocessor systems motivates the following graph-the
oretic definition. A subset C of points in an undirected graph G = (V, E) i
s called an identifying code if the sets B(upsilon) boolean AND C consistin
g of all elements of C within distance one from the vertex upsilon are diff
erent. We also require that the sets B(upsilon) boolean AND C are all nonem
pty. We take G to be the infinite square lattice with diagonals and show th
at the density of the smallest identifying code is at least 2/9 and at most
4/17.