  We have developed a series expansion method for calculating the
zero-temperature properties of lattice electron models for variable electron
density, i.e. for finite doping away from the half-filled case. This is done by
introducing particle fluctuation terms in both the unperturbed Hamiltonian and
perturbation. The method is demonstrated by application to the 2-chain $t-J$
ladder, where we provide comparison with previous work as well as obtaining a
number of new results.
