The Cauchy problem for first order system $L(t,x,{\partial }_t,{\partial }_x)$ is known to be well-posed in $L^2$ when it admits a microlocal symmetrizer $S(t,x,\xi )$ which is smooth in $\xi$ and Lipschitz continuous in $(t,x)$. This paper contains three main results. First we show that a Lipschitz smoothness globally in $(t,x,\xi )$ is sufficient. Second, we show that the existence of symmetrizers with a given smoothness is equivalent to the existence of full symmetrizers having the same smoothness. This notion was first introduced in [FL67]. This is the key point...

Sur ${ℂ}^n$ vu comme variété algébrique, soient $ℱ$ la transformation de Fourier pour les $𝒟$-modules, ${ℱ}^{+}$ la transformation de Fourier faisceautique de Brylinsky-Malgrange-Verdier, et $𝒮ol$ le foncteur “solutions”. On prouve alors que pour tout $𝒟$-module 1-spécialisable à l’infini $ℳ$, on a un isomorphisme $𝒮ol\left(ℱℳ\right)\equiv {ℱ}^{+}𝒮ol\left(ℳ\right)$. Le résultat a été conjecturé en 1988 par B. Malgrange, qui l’a prouvé pour $ℳ$ module de type fini sur l’algèbre de Weyl.

Let $X\cong C^n$, let $M$ be a $C^2$ hypersurface of $X$, $S$ be a $C^2$ submanifold of $M$. Denote by $L_M$ the Levi form of $M$ at $z_0\in S$. In a previous paper [3] two numbers $s^{\pm }\left(S,p\right)$, $p\in {\left({\dot{T}}_S^{*}X\right)}_{z_0}$ are defined; for $S=M$ they are the numbers of positive and negative eigenvalues for $L_M$. For $S\subset M$, $p\in S\times {}_M\dot{T}{}^{*}{}_{S}X)$, we show here that $s^{\pm }\left(S,p\right)$ are still the numbers of positive and negative eigenvalues for $L_M$ when restricted to $T_{z_0}^{C}S$. Applications to the concentration in degree for microfunctions at the boundary are given.

We discuss existence of global solutions of moderate growth to a linear partial differential equation with constant coefficients whose total symbol P(ξ) has the origin as its only real zero. It is well known that for such equations, global solutions tempered in the sense of Schwartz reduce to polynomials. This is a generalization of the classical Liouville theorem in the theory of functions. In our former work we showed that for infra-exponential growth the corresponding assertion is true if and...