New Fundamental Symmetries of Integrable Systems and Partial Bethe Ansatz

We introduce a new concept of quasi-Yang-Baxter algebras. The quantum quasiYang-Baxrer algebras being simple but non-trivial deformations of ordinary algebras of monodromy matrices realize a new type of quantum dynamical symmetries and nd an unexpected and remarkable applications in quantum inverse scattering method (QISM). We show that applying to quasi-Yang-Baxter algebras the standard procedure of QISM one obtains new wide classes of quantum models which, being integrable (i.e. having enough number of commuting integrals of motion) are only quasi-exactly solvable (i.e. admit an algebraic Bethe ansatz solution for arbitrarily large but limited parts of the spectrum). These quasi-exactly solvable models naturally arise as deformations of known exactly solvable ones. A general theory of such deformations is proposed. The correspondence \Yangian | quasi-Yangian" and \XXX spin models | quasi-XXX spin models" is discussed in detail. We also construct the classical conterparts of quasi-Yang-Baxter algebras and show that they naturally lead to new classes of classical integrable models. We conjecture that these models are quasi-exactly solvable in the sense of classical inverse scattering method, i.e. admit only partial construction of action-angle variables. This work was partially supported by DFG grant No. 436 POL 113/77/0 (S). E-mail address: [email protected] and [email protected]