$L^p-L^q$ estimates for maximal operators associated to families of finite type curves

Abstract

We study the boundedness problem for maximal operators $\mathbb{M}$ associated to averages along families of finite type curves in the plane, defined by $$\mathbb{M}f(x) \, := \, \sup_{1 \leq t \leq 2} \left|\int_{\mathbb{C}} f(x-ty) \, \rho(y) \, d\sigma(y)\right|,$$ where $d\sigma$ denotes the normalised Lebesgue measure over the curves $\mathbb{C}$. Let $\triangle$ be the closed triangle with vertices $P=(\frac{2}{5}, \frac{1}{5}), ~ Q=(\frac{1}{2}, \frac{1}{2}), ~ R=(0, 0).$ In this paper, we prove that for $(\frac{1}{p}, \frac{1}{q}) \in (\triangle \setminus \{P, Q\}) \cap \left\{(\frac{1}{p}, \frac{1}{q}) :q > m \right\}$, there is a constant $B$ such that $\|\mathbb{M}f\|_{L^q(\mathbb{R}^2)} \leq \, B \, \|f\|_{L^p(\mathbb{R}^2)}$. Furthermore, if $m <5,$ then we have $\|\mathbb{M}f\|_{L^{5, \infty}(\mathbb{R}^2)} \leq B \|f\|_{L^{\frac{5}{2} ,1} (\mathbb{R}^2)}.$ We shall also consider a variable coefficient version of maximal theorem and we obtain the $L^p-L^q$ boundedness result for $ (\frac{1}{p}, \frac{1}{q}) \in \triangle^{\circ} \cap \left\{(\frac{1}{p}, \frac{1}{q}) :q > m \right\},$ where $\triangle^{\circ}$ is the interior of the triangle with vertices $(0,0), ~(\frac{1}{2}, \frac{1}{2}), ~(\frac{2}{5}, \frac{1}{5}).$ An application is given to obtain $L^p-L^q$ estimates for solution to higher order, strictly hyperbolic pseudo-differential operators.