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Lower bounds on Ricci curvature and quantitative behavior of singular sets. (English) Zbl 1268.53053
Authors’ summary: Let $$Y^n$$ denote the Gromov-Hausdorff limit $$M^n_i \overset {d_{\text{GH}}} {\longrightarrow} Y^n$$ of v-noncollapsed Riemannian manifolds with $$\text{Ric}_{M^n_i}\geq -(n-1)$$. The singular set $$\mathcal {S}\subset Y$$ has a stratification $$\mathcal {S}^0\subset \mathcal {S}^{1}\subset\cdots\subset \mathcal {S}$$, where $$y\in \mathcal {S}^k$$ if no tangent cone at $$y$$ splits off a factor $$\mathbb R^{k+1}$$ isometrically. Here, we define for all $$\eta >0$$, $$0<r\leq 1$$, the “$$k$$-th effective singular stratum” $$\mathcal {S}^k_{\eta, r}$$ satisfying $$\bigcup_\eta\bigcap_r\mathcal {S}^k_{\eta, r}= \mathcal {S}^{k}$$. Sharpening the known Hausdorff dimension bound $$\dim \mathcal {S}^{k}\leq k$$, we prove that for all $$y$$, the volume of the $$r$$-tubular neighborhood of $$\mathcal {S}^{k}_{\eta,r}$$ satisfies $$\text{Vol}(T_r(\mathcal {S}^{k}_{\eta,r})\cap B_{\frac{1}{2}}(y))\leq c(n,\text{v},\eta)r^{n-k-\eta}$$. The proof involves a “quantitative differentiation argument”. This result has applications to Einstein manifolds. Let $$\mathcal {B}_r$$ denote the set of points at which the $$C ^2$$-harmonic radius is $$\leq r$$. If also the $$M^n_i$$ are Kähler-Einstein with $$L _{2}$$ curvature bound, $$\|Rm\|_{L_2}\leq C$$, then $$\text{Vol}( \mathcal {B}_r\cap B_{\frac{1}{2}}(y))\leq c(n,{\text{v}},C)r^{4}$$ for all $$y$$. In the Kähler-Einstein case, without assuming any integral curvature bound on the $$M^n_i$$, we obtain a slightly weaker volume bound on $$\mathcal {B}_r$$ which yields an a priori $$L _p$$ curvature bound for all $$p<2$$. The methodology developed in this paper is new and is applicable in many other contexts. These include harmonic maps, minimal hypersurfaces, mean curvature flow and critical sets of solutions to elliptic equations.

##### MSC:
 53C23 Global geometric and topological methods (à la Gromov); differential geometric analysis on metric spaces 58A35 Stratified sets
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