EXACT 2-LOOP VACUUM POLARIZATION CORRECTION TO THE LAMB SHIFT IN HYDROGENLIKE IONS

We present a calculation scheme for the two-loop vacuum polarization c orrection of order alpha(2) to the Lamb shift of hydrogenlike high-Z a toms. The interaction with the external Coulomb field is taken into ac count to all orders in (2 alpha). By means of a modified potential app roach the problem is reduced to the evaluation of effective one-loop v acuum polarization potentials. An expression for the energy shift is d educed within the framework of partial wave decomposition performing a ppropriate subtractions. Exact results for the two-loop vacuum polariz ation contribution to the Lamb shift of K-and L-shell electron states in hydrogenlike Lead and Uranium are presented.