2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/145631We prove a Beurling-Helson type theorem on modulation spaces. More precisely, we show that the only $\mathcal{C}^{1}$ changes of variables that leave invariant the modulation spaces $\M{p,q}(\rd)$ are affine functions on $\rd$. A special case of our result involving the Sjöstrand algebra was considered earlier by A. Boulkhemair.Classical Analysis and ODEsFunctional Analysis42B15; 42A45; 42B35text