  The traces over infinite dimensional representations of the central extended
Yangian double for the product of operators which intertwine these
representations are calculated. For the special combinations of the
intertwining operators the traces are identified with form factors of local
operators in SU(2)-invariant Thirring model. This identification is based on
the identities which are deformed analogs of the Gauss-Manin connection
identities for the hyperelliptic curves.
