Norme Minimale sur le Compléxifié d'un Espace de Hilbert Réel

Let \Cal H_{\Bbb R} be a real Hilbert space and let \Cal H_{\Bbb C} be the complexification of \Cal H_{\Bbb R}. The first part of this paper treats the problem of the existence of the minimal norm $\tilde\ell$ on \Cal H_{\Bbb C} such that % \align & \tilde\ell(z)\le\|z\|_{\Cal H_{\Bbb C}}\m\n \hbox{for}\m z\in\Cal H_{\Bbb C} \\ & \tilde\ell(x)=\|x\|_{\Cal H_{\Bbb R}}\m\n \hbox{for}\m x\in\Cal H_{\Bbb R}. \endalign % We prove the following theorem : a)\m The minimal norme $\tilde\ell$ exists in \Cal H_{\Bbb C}. b)\m Let $D\subset\Bbb C^N$ be a bounded, convex, balanced domain. There exists a maximal bounded convex, balanced domain $\tilde D\subset\Bbb C^N$ such that % \tilde D\supset D,\m\n \tilde D\cap\Bbb R^N=D\cap\Bbb R^N. % c)\m Let \Cal H_{\Bbb C}=\Bbb C^N, then the minimal norm $\tilde\ell$ is the supporting function of the unit closed Lie ball in $\Bbb C^N$. (a) and b) extend a result of K. T. Hahn and Peter Plug) where \Cal H_{\Bbb R}=\Bbb R^N and $D$ is the unit euclidean ball in \Cal C^N. The second part of the paper gives a geometrical interpretation of the minimal norm $\tilde\ell$ in \Cal H_{\Bbb C}. If \Cal N is a norm in $\Bbb C^N$, log \Cal N(z) is plurisubharmonic function. The final part of the paper studies the plurisubharmonic functions $V$ in $\Bbb C^N$ such that $\forall k\in\Bbb C$, $V(kz)=|k|V(z)$, $V(z)\le\|z\|$ for $z\in\Bbb C^N$, $V(x)=\|x\|$ for $x\in\Bbb R^N$, $\|z\|$ is euclidean norm in $\Bbb C^N$

Norme Minimale sur le Compléxifié d\u27un Espace de Hilbert Réel

Let \Cal H_{\Bbb R} be a real Hilbert space and let \Cal H_{\Bbb C} be the complexification of \Cal H_{\Bbb R}. The first part of this paper treats the problem of the existence of the minimal norm $\tilde\ell$ on \Cal H_{\Bbb C} such that % \align & \tilde\ell(z)\le\|z\|_{\Cal H_{\Bbb C}}\m\ \hbox{for}\m z\in\Cal H_{\Bbb C} \\ & \tilde\ell(x)=\|x\|_{\Cal H_{\Bbb R}}\m\ \hbox{for}\m x\in\Cal H_{\Bbb R}. \endalign % We prove the following theorem : a)\m The minimal norme $\tilde\ell$ exists in \Cal H_{\Bbb C}. b)\m Let $D\subset\Bbb C^N$ be a bounded, convex, balanced domain. There exists a maximal bounded convex, balanced domain $\tilde D\subset\Bbb C^N$ such that % \tilde D\supset D,\m\ \tilde D\cap\Bbb R^N=D\cap\Bbb R^N. % c)\m Let \Cal H_{\Bbb C}=\Bbb C^N, then the minimal norm $\tilde\ell$ is the supporting function of the unit closed Lie ball in $\Bbb C^N$. (a) and b) extend a result of K. T. Hahn and Peter Plug) where \Cal H_{\Bbb R}=\Bbb R^N and $D$ is the unit euclidean ball in \Cal C^N. The second part of the paper gives a geometrical interpretation of the minimal norm $\tilde\ell$ in \Cal H_{\Bbb C}. If \Cal N is a norm in $\Bbb C^N$, log \Cal N(z) is plurisubharmonic function. The final part of the paper studies the plurisubharmonic functions $V$ in $\Bbb C^N$ such that $\forall k\in\Bbb C$, $V(kz)=|k|V(z)$, $V(z)\le\|z\|$ for $z\in\Bbb C^N$, $V(x)=\|x\|$ for $x\in\Bbb R^N$, $\|z\|$ is euclidean norm in $\Bbb C^N$