In this work we provide a geometric characterization of the measures $\mu$ in $\mathbb R^{ n+1}$ with polynomial upper growth of degree $n$ such that the $n$-dimensional Riesz transform $R\mu (x) = \int \frac{ x-y}{ |x-y|^{ n+1}}\,d\mu(y)$ belongs to $L^2(\mu)$. More precisely, it is shown that $$\|R\mu\|_{ L^2(\mu)}^2 + \|\mu\|\approx \int\!\!\int_0^\infty \beta_{ 2,\mu}(x,r)^2\,\frac{ \mu(B(x,r))}{ r^n}\,\frac{ dr}r\,d\mu(x) + \|\mu\|,$$ where $\beta_{ \mu,2}(x,r)^2 = \inf_L \frac1{ r^n}\int_{ B(x,r)} \left(\frac{ \mathrm{ dist}(y,L)}r\right)^2\,d\mu(y),$ with the infimum taken over all affine $n$-planes $L\subset\mathbb R^{ n+1}$. As a corollary, we obtain a characterization of the removable sets for Lipschitz harmonic functions in terms of a metric-geometric potential and we deduce that the class of removable sets for Lipschitz harmonic functions is invariant by bilipschitz mappings.
Categories: math.CA math.AP