Uniqueness and Zeroth-Order Analysis of Weak Solutions to the Non-cutoff Boltzmann equation
We establish the uniqueness of large solutions to the non-cutoff Boltzmann equation with moderate soft potentials. Specifically, the weak solution $F=\mu+\mu^{\frac{1}{2}}f$ is unique as long as it has finite energy, in the sense that the norm $\|f\|_{L^\infty_t L^{r}_{x,v}}+\|f\|_{L^\infty_t L^2_{x,v}}$ remains bounded for some sufficiently large $r>0$. As a byproduct, we establish $L^2_{t,x,v}$ stability for initial data $f_0\in L^r_{x,v}\cap L^2_{x,v}$. Our approach employs dilated dyadic decompositions in phase space $(v,\xi,\eta)$ to capture hypoellipticity and to reduce the fractional derivative structure $(-\Delta_v)^{s}$ of the Boltzmann collision operator to zeroth order. The difficulties posed by the large solution are overcome through the negative-order hypoelliptic estimate that gains integrability in $(t,x)$.