Series Expansions for Variable Electron Density

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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.