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Given a smooth bounded open subset $\Omega$ of $\mathbb{R}^n$, does there exist $A >0$ such that if $f\in BV(\Omega)$ with zero trace on $\partial \Omega$, and $\int_\Omega |Df| = 1$, then $\|f\|_{L^1(\Omega)} \leq A$? This is not a homework problem.

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    Yes,$A$is independent of f2012-04-11

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Let $\epsilon > 0$ and consider the set $ \Omega_\epsilon = \{x\in \Omega : \text{dist}(x,\partial \Omega) > \epsilon\}. $ with characteristic function $\chi_{\Omega_\epsilon}$. By the coarea formula the function $f_\epsilon = \frac{1}{\mathcal{H}^{n-1}(\partial \Omega_\epsilon)} \cdot \chi_{\Omega_\epsilon}$ has zero trace and fulfils $\int_\Omega|Df_\epsilon| = 1$ and $|| f_\epsilon ||_{L^1(\Omega)} = \frac{\lambda(\Omega_\epsilon)}{\mathcal{H}^{n-1}(\partial \Omega_\epsilon)}$, where $\lambda$ denotes the Lebesgue measure on $\mathbb{R}^n$ and $\mathcal{H}^{n-1}$ the $(n-1)$-dimensional Hausdorff measure. By letting $\epsilon \rightarrow 0$ you get the bound $A = \frac{\lambda(\Omega)}{\mathcal{H}^{n-1}(\partial \Omega)}$.

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    @begiestzwirst: For $f \in BV(\Omega)$ with trace $0$ on the boundary, there exists $g \in C^\infty(\Omega)$ with $g \equiv 0$ on $\partial \Omega$, $\|g\|_{L^1(\Omega)} \geq \frac{1}{2}\|f\|_{L^1(\Omega)}$ and $\int_\Omega |\nabla g| \leq 2\int_\Omega |Df|$, by a theorem of Giusti. Then you use Holder's inequality and a Poincare inequality to get $\|f\|_{L^1(\Omega)} \leq 2\|g\|_{L^1(\Omega)} \leq A\|g\|_{L^\frac{n}{n-1}(\Omega)} \leq B\|\nabla g\|_{L^1(\Omega)} \leq 2B\int_\Omega |Df|$2012-04-12