2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/71664We prove a sharp Hölder estimate for solutions of linear two-dimensional, divergence form elliptic equations with measurable coefficients, such that the matrix of the coefficients is symmetric and has {\em unit determinant}. Our result extends some previous work by Piccinini and Spagnolo. The proof relies on a sharp Wirtinger type inequality.A mistake corrected; references updatedAnalysis of PDEs35J60text