“…As was already mentioned, the expression for the induced vacuum energy per unit area of the boundary surface, see (35), can be regarded as purely formal, since it is ill-defined due to the divergence of infinite sums over l and n. To tame the divergence, a factor containing a regularization parameter should be inserted in (35). A summation over values of k l , which are determined by (20)-(22), is to be performed with the use of versions of the Abel-Plana formula, that were derived in [4,5,14,15]. According to these versions, a contribution of the boundaries is separated into a piece which is free from divergences, and, therefore, the regularization can be safely removed in this piece.…”