q-Variationen des Hörmander-Funktionalkalküls und Schrödinger- und Wellen-Maximalabschätzungen
q-variational Hörmander functional calculus and Schrödinger and wave maximal estimates
q-variations du calcul fonctionnel de Hörmander et estimations maximales de Schrödinger et des ondes
Résumé
This article is the continuation of the work [DK] where we had proved maximal estimates
\[ ||\sup_{t > 0} |m(tA)f| \: ||_{L^p(\Omega,Y)} \leq C ||f||_{L^p(\Omega,Y)} \]
for sectorial operators $A$ acting on $L^p(\Omega,Y)$ ($Y$ being a UMD lattice) and admitting a H\"ormander functional calculus
(a strengthening of the holomorphic $H^\infty$ calculus to symbols $m$ differentiable on $(0,\infty)$ in a quantified manner), and $m : (0, \infty) \to \C$ being a H\"ormander class symbol with certain decay at $\infty$.
In the present article, we show that under the same conditions as above, the scalar function $t \mapsto m(tA)f(x,\omega)$ is of finite $q$-variation with $q > 2$, a.e. $(x,\omega)$.
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
\[ ||\sup_{t > 0} |m(tA) f_t| \: ||_{L^p(\Omega,Y)} \leq C \norm||f_t||_{L^p(\Omega,Y(\Lambda^\beta))} \]
for the same $A$ and similar conditions on $m$ as above but with $f_t$ depending itself on $t$ such that $t \mapsto f_t(x,\omega)$ belongs to a Sobolev space $\Lambda^\beta$ over $(\R_+, \frac{dt}{t})$.
We apply this to show a maximal estimate of the Schr\"odinger (case $A = -\Delta$) or wave (case $A = \sqrt{-\Delta}$) solution propagator $t \mapsto \exp(itA)f$.
Then we deduce from it variants of Carleson's problem of pointwise convergence [Car]
\[ \exp(itA)f(x,\omega) \to f(x,\omega) \text{ a. e. }(x,\omega) \quad (t \to 0+)\]
for $A$ a Fourier multiplier operator or a differential operator on an open domain $\Omega \subseteq \R^d$ with boundary conditions.
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