2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/153800We study zero temperature correlation functions of the spin-$1\over 2$ Heisenberg XXZ model in the critical regime $-1< Δ\leq 1$ in a magnetic field by means of the {\tenit Dual Field Approach}. We show for one particular example how to derive determinant representations for correlation functions and how to use these to embed the correlation functions in integrable systems of integro-difference equations (IDE). These IDE are associated with a Riemann-Hilbert problem.19 pages LaTeX, to appear in the proceedings of SMQFT, Los Angeles 1994Exactly Solvable and Integrable Systemstext