  We discuss the quantization of the energy levels of two-particle scattering
states in a finite volume in the case of Bloch-type boundary conditions. A
generalization of the Luescher quantization condition is obtained that can be
used in order to calculate the scattering phases, resulting for example by the
strong elastic interaction of two pions, at fixed physical momentum transfers
on a sequence of volumes of growing sizes. We also give a generalization of the
Lellouch-Luescher formula to be used to extract the physical decay rate for a
kaon decaying into two pions below the inelastic threshold from finite volume
calculations. The formula is valid up to corrections exponentially vanishing in
the volume. By using this formula the calculation can in principle be performed
on different finite volumes of growing sizes in order to keep under control the
corrections.
