  We study one-dimensional insulators obeying a chiral symmetry in the
single-particle picture. The Fermi level is assumed to lie in a mobility gap.
Topological indices are defined for infinite (bulk) or half-infinite (edge)
systems, and it is shown that for a given Hamiltonian with nearest neighbor
hopping the two indices are equal. We also give a new formulation of the index
in terms of the Lyapunov exponents of the zero energy Schr\"odinger equation,
which illustrates the conditions for a topological phase transition occurring
in the mobility gap regime.
