2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/94459We prove that the tau-function of the integrable discrete sine-Gordon model apart from the "standard" bilinar identities obeys a number of "non-standard" ones. They can be combined into a bivector 3-dimensional difference equation which is shown to contain Hirota's difference analogue of the sine-Gordon equation and both auxiliary linear problems for it. We observe that this equation is most naturally written in terms of the quantum R-matrix for the XXZ spin chain and looks then like a relation of the "vertex-face correspondence" type.14 pages, latexExactly Solvable and Integrable SystemsHigh Energy Physics - Theorytext