2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/18689We 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.11 figuresStrongly Correlated Electronstext