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Area and Gauss-Bonnet inequalities with scalar curvature

Published 14 Dec 2021 in math.DG | (2112.07245v1)

Abstract: Let $X$ be an $n$-dimensional Riemannian manifold with "large positive" scalar curvature. In this paper, we prove in a variety of cases that if $X$ "spreads" in $(n-2)$ directions {\it "distance-wise"}, then it {\it can't} much "spread" in the remaining 2-directions {\it "area-wise".} Here is a geometrically transparent example of what we plan prove in this regard that illustrates the idea. Let $g$ be a Riemannin metric on $X= S2\times \mathbb R{n-2}$, for which the submanifolds $$\mbox {$\mathbb R_s{n-2}=s\times \mathbb R{n-2}\subset X$ and $S2_y= S2\times y \subset X$}$$ are {\it mutually orthogonal} at all intersection points $$x=(s,y)\in X=\mathbb R_s{n-2}\cap S2_y.$$ (An instance of this is $g=g(s,y)=\phi(s,y)2ds2+\psi(s,y)2dy2$.) Let the Riemannian metric on $\mathbb R_s{n-2}$ induced from $(X,g)$, that is $g|_{\mathbb R_s{n-2}}$, be {\it greater than the Euclidean} metric on $\mathbb R_s{n-2} =\mathbb R{n-2}$ for all $s\in S2$. (This is interpreted as "large spread" of $g$ in the $(n-2)$ Euclidean directions.) {\sf If the {\it scalar curvature of $g$ is strictly greater than that of the unit 2-sphere}, $$Sc(g) \geq Sc(S2)+\varepsilon=2+\varepsilon, \mbox { }\varepsilon>0,$$ then, provided $n\leq 7$, } (this, most likely, is unnecessary) {\sf there exists a smooth {\it non-contractible} spherical surface $S\subset X$, such that $$area(S)<area(S2)=4\pi.$$} (This says, in a way, that $(X,g)$ "doesn't spread much area-wise" in the 2 directions complementary to the Euclidean ones.)

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