Bernstein inequality on conic domains and triangle

Abstract: We establish weighted Bernstein inequalities in $L^p$ space for the doubling weight on the conic surface $\mathbb{V}0^{d+1} = {(x,t): |x| = t, x \in \mathbb{R}^d, t\in [0,1]}$ as well as on the solid cone bounded by the conic surface and the hyperplane $t =1$, which becomes a triangle on the plane when $d=1$. While the inequalities for the derivatives in the $t$ variable behave as expected, there are inequalities for the derivatives in the $x$ variables that are stronger than what one may have expected. As an example, on the triangle ${(x_1,x_2): x_1 \ge 0, \, x_2 \ge 0,\, x_1+x_2 \le 1}$, the usual Bernstein inequality for the derivative $\partial_1$ states that $|\phi_1 \partial_1 f|{p,w} \le c n |f|{p,w}$ with $\phi_1(x_1,x_2):= x_1(1-x_1-x_2)$, whereas our new result gives $$| (1-x_2)^{-1/2} \phi_1 \partial_1 f|{p,w} \le c n |f|_{p,w}.$$ The new inequality is stronger and points out a phenomenon unobserved hitherto for polygonal domains.