If $U$ is a $C^{ \infty}$ function with compact support in the plane, we let $u$ be its restriction to the unit circle $\mathbb{ S}$, and denote by $U_i,\,U_e$ the harmonic extensions of $u$ respectively in the interior and the exterior of $\mathbb S$ on the Riemann sphere. About a hundred years ago, Douglas has shown that \begin{ align*} \iint_{ \mathbb{ D}}|\nabla U_i|^2(z)dxdy&= \iint_{ \bar{ \mathbb{ C}}\backslash\bar{ \mathbb{ D}}}|\nabla U_e|^2(z)dxdy &= \frac{ 1}{ 2\pi}\iint_{ \mathbb S\times\mathbb S}\left|\frac{ u(z_1)-u(z_2)}{ z_1-z_2}\right|^2|dz_1||dz_2|, \end{ align*} thus giving three ways to express the Dirichlet norm of $u$. On a rectifiable Jordan curve $\Gamma$ we have obvious analogues of these three expressions, which will of course not be equal in general. The main goal of this paper is to show that these $3$ (semi-)norms are equivalent if and only if $\Gamma$ is a chord-arc curve.
Categories: math.CV math.CA