MathOverflow

Let $BH(\mathbb{R}^2)$ be the space of distributions $u$ such that $\nabla^2 u$ is a matrix-valued Radon measure, with norm $\|u\|_{BH} = \|\nabla^2 u\|_{\mathcal{M}(\mathbb{R}^2)^{2 \times 2}}$.
Consider the unit ball $\mathcal{B}_{BH} = \{u : \|u\|_{BH} \le 1\}$....

Note: I'm actively learning this material, so if I say anything inaccurate or confusing, please feel free to leave a comment.
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