2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/153027Hypoellipticity in Gevrey classes $G^s$ is characterized for a simple family of sums of squares of vector fields satisfying the bracket hypothesis, with analytic coefficients. It is shown that hypoellipticity holds if and only if $s$ is greater than or equal to an optimal exponent that may take on any rational value.Functional Analysistext