  An analytical method to compute the LDOS energy spectrum and stationary
states for finite size doped monatomic chains modelled by an effective
one-dimensional tight-binding hamiltonian is presented. It is based on the
formal solution of linear second order recurrence relations. We also study the
LDOS energy spectrum of some doped monatomic chains applying a perturbative
approach to the characteristic equation of the reference metallic structure
doped with a few extraneous atoms.
