We study the regularity properties of the second order linear operator in $ {{\mathbb {R}}}^{N+1} $:
$ \begin{equation*} \mathscr{L} u : = \sum\limits_{j,k = 1}^{m} a_{jk}\partial_{x_j x_k}^2 u + \sum\limits_{j,k = 1}^{N} b_{jk}x_k \partial_{x_j} u - \partial_t u, \end{equation*} $
where $ A = \left( a_{jk} \right)_{j,k = 1, \dots, m}, B = \left( b_{jk} \right)_{j,k = 1, \dots, N} $ are real valued matrices with constant coefficients, with $ A $ symmetric and strictly positive. We prove that, if the operator $ {\mathscr{L}} $ satisfies Hörmander's hypoellipticity condition, and $ f $ is a Dini continuous function, then the second order derivatives of the solution $ u $ to the equation $ {\mathscr{L}} u = f $ are Dini continuous functions as well. We also consider the case of Dini continuous coefficients $ a_{jk} $'s. A key step in our proof is a Taylor formula for classical solutions to $ {\mathscr{L}} u = f $ that we establish under minimal regularity assumptions on $ u $.
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