most recent 30 from http://mathoverflow.net 2013-05-26T04:06:48Z http://mathoverflow.net/feeds/question/72003 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/72003/convergence-of-elliptic-operators Chris Judge 2011-08-03T16:15:29Z 2011-08-03T17:32:24Z

<p>Let $A_t$ be family of second order, positive, elliptic differential operator mapping Sobolev $H^2$ of a compact smooth manifold (or bounded domain) to L^2. Suppose that the coefficients of $A_t$ converge uniformly in $C^k$ for every $k$ to the coefficients of a second order, positive, elliptic differential operator $A$. $A$ is invertible (with domain L^2 and range H^2) and so we may consider the sequence $A_t \circ A_0^{-1}$ of operators from $L^2$ to $L^2$. Does this family converge to the identity in the $L^2$ operator norm? Why or why not?</p>

http://mathoverflow.net/questions/72003/convergence-of-elliptic-operators/72017#72017 Deane Yang 2011-08-03T17:32:24Z 2011-08-03T17:32:24Z

<p>It suffices to show that the $L_2$ operator norm of $A_t\circ A_0^{-1} - I = (A_t - A_0)\circ A_0^{-1}$ is small if $t$ is sufficiently small. To do this, it suffices to show that the operator norm of $A_t - A_0$, as map from $H^2$ to $L_2$ is small if $t$ is small. But a linear second order operator like this has small operator norm, if the $C^0$ norm of the coefficients are small. So the fact that the coefficients converge in the $C^0$ norm gives what you want.</p>