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The simplest case of the problem I'm thinking about involves an elliptic differential operator, $Lu = -u'' + qu$, on the interval $(0,1)$, with homogeneous Dirichlet boundary conditions. I want to show that the bilinear form on $H_0^1 \subset H_1$ defined by

$a(u,v) = \int_0^1 u'v' + quv~dx$

is bounded for the $H^1$-norm, i.e., $|a(u,v)| \leq M\|u\|_1\|v\|_1$ for some constant $M>0$.

My question: can I assume that the linear coefficient $q$ is $L^1$ or even $L^2$ and still guarantee boundedness?

I was thinking that this is possible, but the only books that I have lying around discussing this consider only the case when $q$ is smooth or $L^\infty$. I've played around with the Cauchy-Schwarz inequality for the term $\int quv$ but am not getting anywhere.

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  • $\begingroup$ I think this is just a standard Sobolev inequality... $\endgroup$
    – Helge
    Aug 13, 2010 at 18:25

1 Answer 1

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Of course, as Helge says, the $H^1(0,1)$ norm controls the $L^\infty(0,1)$ norm, so you just write $$ \left| \int quv \right| \le \|q\|_{L^1} \|u\| _{L^\infty} \|v\| _{L^\infty} \le C \|q\| _{L^1} \|u\| _{H^1} \|v\| _{H^1}.$$

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  • $\begingroup$ And to control the $L^2$ case, use this estimate once and C-S on $\langle q, u \rangle$. $\endgroup$
    – Helge
    Aug 13, 2010 at 21:30
  • $\begingroup$ Right, thanks! I was hoping it would be that straightforward. Was just suffering from temporary myopia. $\endgroup$
    – Jerry
    Aug 13, 2010 at 22:36

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