Multidimensional bilinear Hardy inequalities

Abstract: Our goal in this paper is to find a characterization of $n$-dimensional bilinear Hardy inequalities \begin{align*} \bigg| \,\int_{B(0,\cdot)} f \cdot \int_{B(0,\cdot)} g \,\bigg|{q,u,(0,\infty)} & \leq C \, |f|{p_1,v_1,{\mathbb R}^n} \, |g|{p_2,v_2,{\mathbb R}^n}, \quad f,\,g \in {\mathfrak M}^+ ({\mathbb R}^n), \end{align*} and \begin{align*} \bigg| \,\int{\,^{^{\mathsf{c}}}! B(0,\cdot)} f \cdot \int_{\,^{^{\mathsf{c}}}! B(0,\cdot)} g \,\bigg|{q,u,(0,\infty)} &\leq C \, |f|{p_1,v_1,{\mathbb R}^n} \, |g|{p_2,v_2,{\mathbb R}^n}, \quad f,\,g \in {\mathfrak M}^+ ({\mathbb R}^n), \end{align*} when $0 < q \le \infty$, $1 \le p_1,\,p_2 \le \infty$ and $u$ and $v_1,\,v_2$ are weight functions on $(0,\infty)$ and ${\mathbb R}^n$, respectively. Since the solution of the first inequality can be obtained from the characterization of the second one by usual change of variables we concentrate our attention on characterization of the latter. The characterization of this inequality is easily obtained for the range of parameters when $p_1 \le q$ using the characterizations of multidimensional weighted Hardy-type inequalites while in the case when $q < p_1$ the problem is reduced to the solution of multidimensional weighted iterated Hardy-type inequality. To achieve the goal, we characterize the validity of multidimensional weighted iterated Hardy-type inequality $$ \left|\left|\int{\,^{^{\mathsf{c}}}! B(0,\cdot)}h(z)dz\right|{p,u,(0,t)}\right|{q,\mu,(0,\infty)}\leq c |h|_{\theta,v,(0,\infty)},~ h \in \mathfrak{M}^+({\mathbb R}^n) $$ where $0 < p,\,q < +\infty$, $1 \leq \theta \le \infty$, $u\in {\mathcal W}(0,\infty)$, $v \in {\mathcal W}({\mathbb R}^n)$ and $\mu$ is a non-negative Borel measure on $(0,\infty)$.