  A perturbative formula for the lowest Lyapunov exponent of an Anderson model
on a strip is presented. It is expressed in terms of an energy dependent doubly
stochastic matrix, the size of which is proportional to the strip width. This
matrix and the resulting perturbative expression for the Lyapunov exponent are
evaluated numerically. Dependence on energy, strip width and disorder strength
are thoroughly compared with the results obtained by the standard transfer
matrix method. Good agreement is found for all energies in the band of the free
operator and this even for quite large values of the disorder strength.
