arXiv2023
We consider degenerate KFP operators \[ Lu=\sum_{i,j=1}^{m_{0}}a_{ij}(x,t)\partial_{x_{i}x_{j}}^{2}u+\sum_{k,j=1}^{N}b_{jk}x_{k}\partial_{x_{j}}u-\partial_{t}u\equiv\sum_{i,j=1}^{m_{0}}a_{ij}(x,t)\partial_{x_{i}x_{j}}^{2}u+Yu \] ($(x,t)\in\mathbb{R}^{N+1}$, $1\leq m_{0}\leq N$) s.t. the model operator having constant $a_{ij}$ is hypoelliptic, translation invariant w.r.t. a Lie group in $\mathbb{R}^{N+1}$ and $2$-homogeneous w.r.t. a family of dilations; $(a_{ij})_{i,j=1}^{m_{0}}$ is symmetric and uniformly positive on $\mathbb{R}^{m_{0}}$; $a_{ij}$ are bounded and Dini continuous in space, bounded measurable in time, i.e.: setting \[ S_{T}=\mathbb{R}^{N}\times\left( -\infty,T\right) , \] \[ ω_{f,S_{T}}(r)=\sup_{\substack{(x,t),(y,t)\in S_{T}\\\Vert x-y\Vert\leq r}}|f(x,t)-f(y,t)| \] \[ \Vert f\Vert_{\mathcal{D}(S_{T})}=\int_{0}^{1}\frac{ω_{f,S_{T}}(r)}% {r}dr+\Vert f\Vert_{L^{\infty}\left( S_{T}\right) } \] we ask $\Vert a_{ij}\Vert_{\mathcal{D}(S_{T})}<\infty$. We bound $ω_{u_{x_{i}x_{j}},S_{T}}$, $\left\Vert u_{x_{i}x_{j}}\right\Vert _{L^{\infty}(S_{T})}$ ($i,j=1,2,...,m_{0}$), $ω_{Yu,S_{T}}$, $\Vert Yu\Vert_{L^{\infty}(S_{T})}$ in terms of $ω_{\mathcal{L}u,S_{T}}$, $\Vert Lu\Vert_{L^{\infty}(S_{T})}$ and $\Vert u\Vert_{L^{\infty}\left( S_{T}\right) }$, getting a control on the uniform continuity in space of $u_{x_{i}x_{j}},Yu$ if $Lu$ is bounded and Dini-continuous in space. Under the additional assumption that $a_{ij}$ and $\mathcal{L}u$ are log-Dini continuous, meaning the finiteness of the quantity% \[ \int_{0}^{1}\frac{ω_{f,S_{T}}\left( r\right) }{r}\left\vert \log r\right\vert dr, \] we prove that $u_{x_{i}x_{j}}$ and $Yu$ are Dini continuous; moreover, in this case, the derivatives $u_{x_{i}x_{j}}$ are locally uniformly continuous in space and time.