Multipliers in Bessel potential spaces. The case of different sign smooth indices

Abstract

The objective of this paper is to describe the space of multipliers acting from a Bessel potential space $H^s_p(\mathbb R^n)$ into another space $H^{-t}_q(\mathbb R^n)$, provided that the smooth indices of these spaces have different signs, i.e. $s, t \geqslant 0$. This space of multipliers consists of distributions $u$, such that for all $\varphi \in H^s_p(\mathbb R^n)$ the product $\varphi \cdot u$ is well-defined and belongs to the space $H^{-t}_q(\mathbb R^n)$. We succeed to describe this space explicitly, provided that $p \leqslant q$ and one of the following conditions $$ s \geqslant t \geqslant 0, \ s > n/p \ \ \, \text{or} \ \ \, t \geqslant s \geqslant 0, \ t > n/q' \quad (\: \text{where} \; 1/q +1/q' = 1), $$ holds. In this case one has $$ M[H^s_p(\mathbb{R}^n) \to H^{-t}_{q}(\mathbb{R}^n)] = H^{-t}_{q, \: unif}(\mathbb{R}^n) \cap H^{-s}_{p', \: unif}(\mathbb{R}^n), $$ where $H^\gamma_{r, \: unif}(\mathbb{R}^n), \: \gamma \in \mathbb{R}, \: r > 1$ is the scale of uniformly localized Bessel potential spaces. In particular but important case $s = t < n/\max (p,q')$ we prove two-sided continuous embeddings $$ H^{-s}_{r_1, \: unif}(\mathbb{R}^n) \subset M[H^s_p(\mathbb{R}^n) \to H^{-s}_q(\mathbb{R}^n)] \subset H^{-s}_{r_2, \: unif}(\mathbb{R}^n), $$ where $r_2 = \max (p', q), \ r_1 =[s/n-(1/p -1/q)]^{-1}$.