2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/73143We give an explicit formula for the symbol of a function of an operator. Given an operator {\hat A} on L^2(R^N) with symbol A, and a function f, we obtain the symbol of f(\hat A) in terms of A. As an application, Bohr-Sommerfeld quantization rules are explicitely calculated at order 4 in hbar.26 pages. v2 contains a better, shorter proof. The problem is considered in more generality. References are correctedQuantum AlgebraMathematical Physics53D55 (primary), 81S10 (secondary)text