$\varepsilon$-regularity criteria in anisotropic Lebesgue spaces and regularity in one direction to the 3D Navier-Stokes equations

In this paper, we derive some $\varepsilon$-regularity criteria in anisotropic Lebesgue spaces for suitable weak solutions to the 3D Navier-Stokes equations as follows \begin{align} &\limsup\limits_{\varrho\rightarrow0} \varrho^{1-\frac{2}{p}-\frac{1}{p}-\frac{1}{q}-\frac{1}{r}} \|u \|_{L_{t}^{p}L_{1}^{q}L_{2}^{r}L_{3}^{s}(Q(\varrho))} \leq\varepsilon,~~~2/p+1/q+1/r+1/s\leq2,~~\text{with}~q,r,s>2; \nonumber &\limsup\limits_{\varrho\rightarrow0}\varrho^{1-\frac{2}{p}-\frac{1}{q}-\frac{1}{r}} \|\nabla_{1}u \|_{L_{t}^{p}L_{1}^{1}L_{2}^{r}L_{3}^{s}(Q(\varrho))} \leq\varepsilon,~~~2/p +2/r+1/s\leq2,~~\text{with}~ r,s>2. \end{align} which extends previous corresponding results by Tian and Xin in [29, Comm. Anal. Geom. 7: 221--257, 1999] and by Gustafson, Kang and Tsai, [14, Commun. Math. Phys. 273: 161--176, 2007]. As a by-product, this allows us to obtain local regularity criteria in terms of $\nabla_{1}u$, namely, $$ \nabla_{1}u\in L^{p}_{t}L^{q}_{x_{1}}L^{r}_{x_{2}}L^{s}_{x_{3}}(Q(\varrho))\leq\varepsilon, ~~~~ \text{with} ~~~ 2/p+1/q+1/r+1/s=2, ~~~2<q,r,s\leq3, 2\leq p<4.$$ This generalizes the recent result by Kukavica, Rusin and Ziane [20, J. Nonlinear Sci. 27: 1725-1742, 2017], where the range of $(p,q=r=s)$ is that $\frac{9}{4}<q<3$ and $2<p<3$. More importantly, the proof utilized in (0.1) together with the result of [14] implies \begin{equation} \limsup\limits_{\varrho\rightarrow0}\varrho^{2- \frac 2p -\frac 3q} \|\nabla_{1}u\|_{L_{t}^{p}L_{x}^{q}(Q(\varrho))}\leq\varepsilon, \,\,2\le 2/p +3/q \le 3, \; 1 \le p,q \le \infty. \end{equation} This gives an improvement of corresponding result obtained by Caffarelli, Kohn and Nirenberg in [1] and in [14]. It is worth remarking that (0.3) yields full range of $q$ in (0.2) with $q=r=s.$ This is of independent interest.