  We present a novel and rigorous approach to the Langevin dynamics of ideal
polymer chains subject to internal distance constraints. The permanent
constraints are modelled by harmonic potentials in the limit when the strength
of the potential approaches infinity (hard crosslinks). The crosslinks are
assumed to exist between arbitrary pairs of monomers. Formally exact
expressions for the resolvent and spectral density matrix of the system are
derived. To illustrate the method we study the diffusional behavior of monomers
in the vicinity of a single crosslink within the framework of the Rouse model.
The same problem has been studied previously by Warner (J. Phys. C: Solid State
Phys. {\bf 14}, 4985, (1981)) on the basis of Lagrangian multipliers. Here we
derive the full, hence exact, solution to the problem.
