Second order Sobolev regularity for normalized parabolic $p(x)$-Laplace equations via the algebraic structure

Denote by $\Delta$ the Laplacian and by $\Delta_\infty$ the $\infty$-Laplacian. A fundamental inequality is proved for the algebraic structure of $\Delta v\Delta_\infty v$: for every $v\in C^{\infty}$, $$\bigg| |D^2vDv|^2-\Delta v\Delta_\infty v-\frac{1}{2}[|D^2v|^2-(\Delta v)^2]|Dv|^2\bigg| \le\frac{n-2}{2}[|D^2v|^2|Dv|^2-|D^2vDv|^2]$$ Based on this, we prove the result: When $n\ge2$ and $p(x)\in(1,2)\cup(2,3+\frac{2}{n-2})$, the viscosity solutions to parabolic normalized $p(x)$-Laplace equation have the $W^{2,2}_{loc}$-regularity in the spatial variable and the $W^{1,2}_{loc}$-regularity in the time variable.