Exterior Robin boundary is expressed as the following in the book $\partial{u}/ \partial{v}-\lambda u=f$ on the boundary and $v$ is normal. Also u satisfies Laplace Equation in exterior domain in $R^2$
$u=O(1/r)$ when $|x|=r\to\infty$
Here i do not understand why $1/r$ is written. Can we write $1/r^2$ or $1/r^3$ instead of $1/r$ ? We know $1/r^2$ is faster when $r \to \infty$. But i did not see this in the book. Thank you in advance.