$q$-variational H{ö}rmander functional calculus and Schr{ö}dinger and wave maximal estimates

Abstract

This article is the continuation of the work [DK] where we had proved maximal estimates $$\left\|\sup_{t > 0} |m(tA)f| \right\|_{L^p(Ω,Y)} \leq C \|f\|_{L^p(Ω,Y)}$$ for sectorial operators $A$ acting on $L^p(Ω,Y)$ ($Y$ being a UMD lattice) and admitting a Hörmander functional calculus(a strengthening of the holomorphic $H^\infty$ calculus to symbols $m$ differentiable on $(0,\infty)$ in a quantified manner), and $m : (0, \infty) \to \mathbb{C}$ being a Hörmander class symbol with certain decay at $\infty$.In the present article, we show that under the same conditions as above, the scalar function $t \mapsto m(tA)f(x,ω)$ is of finite $q$-variation with $q > 2$, a.e. $(x,ω)$.This extends recent works by [BMSW,HHL,HoMa1,HoMa,JSW,LMX] who have considered among others $m(tA) = e^{-tA}$ the semigroup generated by $-A$.As a consequence, we extend estimates for spherical means in euclidean space from [JSW] to the case of UMD lattice-valued spaces.A second main result yields a maximal estimate $$\left\|\sup_{t > 0} |m(tA) f_t| \right\|_{L^p(Ω,Y)} \leq C \|f_t\|_{L^p(Ω,Y(Λ^β))}$$ for the same $A$ and similar conditions on $m$ as above but with $f_t$ depending itself on $t$ such that $t \mapsto f_t(x,ω)$ belongs to a Sobolev space $Λ^β$ over $(\mathbb{R}_+, \frac{dt}{t})$.We apply this to show a maximal estimate of the Schrödinger (case $A = -Δ$) or wave (case $A = \sqrt{-Δ}$) solution propagator $t \mapsto \exp(itA)f$.Then we deduce from it variants of Carleson's problem of pointwise convergence [Car]\[ \exp(itA)f(x,ω) \to f(x,ω) \text{ a. e. }(x,ω) \quad (t \to 0+)\]for $A$ a Fourier multiplier operator or a differential operator on an open domain $Ω\subseteq \mathbb{R}^d$ with boundary conditions.