Interpolation of nonlinear maps

TL;DR

This paper establishes that analytic maps between complex Banach space couples, which are bounded on endpoint spaces, interpolate to analytic maps on intermediate spaces with controlled bounds.

Contribution

It proves that under certain boundedness conditions, analytic maps between Banach couples interpolate to analytic maps on the complex interpolation spaces.

Findings

Analytic maps on Banach couples interpolate to the intermediate spaces.

Boundedness constants interpolate as geometric means.

Results extend the understanding of analytic map behavior in complex interpolation.

Abstract

Let $(X_0, X_1)$ and $(Y_0, Y_1)$ be complex Banach couples and assume that $X_1\subseteq X_0$ with norms satisfying $\|x\|_{X_0} \le c\|x\|_{X_1}$ for some $c > 0$. For any $0<\theta <1$, denote by $X_\theta = [X_0, X_1]_\theta$ and $Y_\theta = [Y_0, Y_1]_\theta$ the complex interpolation spaces and by $B(r, X_\theta)$, $0 \le \theta \le 1,$ the open ball of radius $r>0$ in $X_\theta$, centered at zero. Then for any analytic map $\Phi: B(r, X_0) \to Y_0+ Y_1$ such that $\Phi: B(r, X_0)\to Y_0$ and $\Phi: B(c^{-1}r, X_1)\to Y_1$ are continuous and bounded by constants $M_0$ and $M_1$, respectively, the restriction of $\Phi$ to $B(c^{-\theta}r, X_\theta)$, $0 < \theta < 1,$ is shown to be a map with values in $Y_\theta$ which is analytic and bounded by $M_0^{1-\theta} M_1^\theta$.