Note on an elementary particle model with Bertotti-Robinson core

Spherically symmetric classical model of an elementary particle or a black hole spacetime without central singularity had been constructed by O. B. Zaslavskii in PRD 70(2004)104017. In this model an extremal Reissner-Nordstr% \"{o}m (RN) black hole and a Bertotti-Robinson (BR) spacetime are glued at the horizon such that the inner/core spacetime is the regular BR while outside is the extremal RN. In this note we investigate the stability of such a particle / regular black hole against linear radial perturbations. The model turns out to be stable against such perturbations with a linear equation of state after the perturbation.