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Topology and ε-regularity Theorems on Collapsed Manifolds with Ricci Curvature Bounds

In this paper we discuss and prove $ε$-regularity theorems for Einstein manifolds $(M^n,g)$, and more generally manifolds with just bounded Ricci curvature, in the collapsed setting. A key tool in the regularity theory of noncollapsed Einstein manifolds is the following: If $x\in M^n$ is such that $Vol(B_1(x))>v>0$ and that $B_2(x)$ is sufficiently Gromov-Hausdorff close to a cone space $B_2(0^{n-\ell},y^*)\subset \mathbb{R}^{n-\ell}\times C(Y^{\ell-1})$ for $\ell\leq 3$, then in fact $|Rm|\leq 1$ on $B_1(x)$. No such results are known in the collapsed setting, and in fact it is easy to see without more such results are false. It turns out that the failure of such an estimate is related to topology. Our main theorem is that for the above setting in the collapsed context, either the curvature is bounded, or there are topological constraints on $B_1(x)$. More precisely, using established techniques one can see there exists $ε(n)$ such that if $(M^n,g)$ is an Einstein manifold and $B_2(x)$ is $ε$-Gromov-Hausdorff close to ball in $B_2(0^{k-\ell},z^*)\subset\mathbb{R}^{k-\ell}\times Z^\ell$, then the fibered fundamental group $Γ_ε(x)\equiv Image[π_1(B_ε(x))\toπ_1(B_2(x))]$ is almost nilpotent with $rank(Γ_ε(x))\leq n-k$. The main result of the this paper states that if $rank(Γ_ε(x))= n-k$ is maximal, then $|Rm|\leq C$ on $B_1(x)$. In the case when the ball is close to Euclidean, this is both a necessary and sufficient condition. There are generalizations of this result to bounded Ricci curvature and even just lower Ricci curvature.