  Starting from an interaction Lagrangian formed out of local fields, an
interacting four-momentum operator is constructed by integrating the
interaction Lagrangian over the forward hyperboloid. Such a four-momentum
operator has the property that the components commute among themselves;
however, when the Fock space on which the four-momentum operator acts is
truncated, the components no longer commute among themselves. By modifying
matrix elements of the four-momentum operator on the truncated space,
Bakamjian-Thomas mass operatorsare constructed which restore the Poincare
relations. Examples for a simple Lagrangian are given.
