Integrable Kondo impurities in the one-dimensional supersymmetric U model of strongly correlated electrons

Integrable Kondo impurities in the one-dimensional supersymmetric U model o f strongly correlated electrons are studied by means of the boundary graded quantum inverse scattering method. The boundary K-matrices depending on th e local magnetic moments of the impurities are presented as non-trivial rea lizations of the reflection equation algebras in an impurity Hilbert space. Furthermore, the model Hamiltonian is diagonalized and the Bethe ansatz eq uations are derived. It is interesting to note that our model exhibits a fr ee parameter in the bulk Hamiltonian but no free parameter exists on the bo undaries. This is in sharp contrast to the impurity models arising from the supersymmetric t-J and extended Hubbard models where there is no free para meter in the bulk but there is a free parameter on each boundary.