Note on linearized stability of Schwarzschild thin-shell wormholes with variable equations of state

We discuss how the assumption of variable equation of state (EoS) allows the elimination of the instability at equilibrium throat radius a(0) = 3M featured by previous Schwarzschild thin-shell wormhole models. Unobstructed stability regions are found for three choices of variable EoS. Two of these EoS entail linear stability at every equilibrium radius. Particularly, the thin shell remains stable as a(0) approaches the Schwarzschild radius 2M. A perturbative analysis of the wormhole equation of motion is carried out in the case of variable Chaplygin EoS. The squared proper angular frequency omega(2)(0) of small throat oscillations is linked with the second derivative of the thin-shell potential. In various situations omega(2)(0) remains positive and bounded in the limit a(0) -> 2M.