# A weak minimum principle.

I am not able to solve the following question that i came across: Let $\Omega \subset R^n$ be bounded. A function $u$ is called a weak solution of the differential inequality $-\Delta u \ge 0$ in $\Omega$ with boundary condition $u=0$ on $\partial\Omega$ if $u\in H_0^1(\Omega)$ and $\int_\Omega \nabla u.\nabla \phi dx \ge 0$ for all $\phi \in H_0^1$ such that $\phi \ge 0$ almost everywhere.

What i want to show is the following : That any such weak solution $u$ satisfies weak minimum principle that $u\ge 0$ a.e in $\Omega$.

Thank you for your help .

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Hint: Try taking $\phi = u^-$, where $u^-(x) = \max\{-u(x), 0\}$.

A more detailed sketch:

• Given any $v \in H^1_0$, show that $$v^- \in H^1_0 \quad \text{and} \quad \nabla v^- = -1_{\{v < 0\}} \nabla v. \quad (*)$$ (This is a good exercise in properties of Sobolev space. Start by considering $v \in C^\infty_0(\Omega)$. Find functions $\psi_n \in C^\infty(\mathbb{R})$ with $\psi_n(s) \to s^-$ in some appropriate sense. Show that $\psi_n \circ v \to v^-$ and $\nabla(\psi_n \circ v) \to -1_{\{v < 0\}} \nabla v$ in $L^2$. This shows (*) holds for $v \in C^\infty_0(\Omega)$. Now given arbitrary $v \in H^1_0$, approximate by $v_n \in C^\infty_0$.)

• It follows from (*) that if $u$ is a weak solution, $$\int_\Omega |\nabla u^-|^2 = -\int_\Omega \nabla u \cdot \nabla u^- \le 0.$$ This means $\nabla u^- = 0$ a.e. and it follows that $u^- = 0$ a.e.

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Sir , i am not able to think much along , it would be nice if you could explain . –  Theorem Jun 17 '12 at 11:39
@Theorem: Which part? –  Nate Eldredge Jul 28 '12 at 22:29
Sir , what does $1_{v<0}\nabla v$ mean ? I am trying to work on with the proof that u have sketched . –  Theorem Jul 29 '12 at 7:20
$1_{\{v < 0\}}$ is the function which is 1 on the set where $v < 0$, and 0 otherwise. In other notation, $$1_{\{v < 0\}}(x) = \begin{cases} 1, & v(x) < 0 \\ 0, & v(x) \ge 0. \end{cases}$$ And $1_{\{v < 0\}} \nabla v$ is just this function multiplied by the gradient of $v$. –  Nate Eldredge Jul 29 '12 at 14:17