Abstract.A new recursion formula is presented for the correlation functions of the integrable spin 1/2 XXX chain with inhomogeneity. It links the correlators involving n consecutive lattice sites to those with n − 1 and n − 2 sites. In a series of papers by V. Korepin and two of the present authors, it was discovered that the correlators have a certain specific structure as functions of the inhomogeneity parameters. The formula mentioned above makes it possible to prove this structure directly, as well as to obtain an exact description of the rational functions that were left undetermined in the earlier work. §1. Introduction
Consider the XXX antiferromagnet given by the HamiltonianThis model was solved in the famous paper by Bethe [3] already in 1931, by using what is now called the coordinate Bethe Ansatz. Nevertheless, it took some time before the physical content of this model in the thermodynamic limit was clarified completely. For the first time, the spectrum of excitations was described correctly in the paper [10] by Faddeev and Takhtajan; it was shown that the spectrum contains magnons of spin 1/2. These authors used the algebraic Bethe Ansatz formulated by Faddeev, Sklyanin, and Takhtajan (see [9]) on the basis of R-matrices and the Yang-Baxter equation. The origin of these new techniques goes back to the work of Baxter [1]. Restricting ourselves to our example, we recall the role of R-matrices in solvable models. The XXX model is related to the rational R-matrix that acts on C 2 ⊗ C 2 (see (2.7) for the explicit formula). We employ the usual notation R 1,2 (λ), where 1, 2 label the corresponding spaces and λ is the spectral parameter. The relationship between the R-matrix and the XXX Hamiltonian is as follows. Consider the transfer matrix