On the classification of rational K-matrices

This paper presents a derivation of the possible residual symmetries of rational K-matrices which are invertible in the 'classical limit' (the spectral parameter goes to infinity). This derivation uses only the boundary Yang–Baxter equation and the asymptotic expansions of the R-matrices. The result proves the previous assumption of the literature: if the original and the residual symmetry algebras are and then there exists a Lie-algebra involution of for which the invariant sub-algebra is . In addition, we study a K-matrix which is not invertible in the 'classical limit'. It is shown that its symmetry algebra is not reductive but a semi-direct sum of reductive and solvable Lie-algebras. (paper)