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Alexandrov geometry studies non smooth analogues of Riemannian manifolds with curvature bounded from below or above. It includes spaces with curvature bounded below (briefly $\mathrm{CBB}[\kappa]$) and spaces with curvature bounded above (briefly $\mathrm{CAT}[\kappa]$).

12 votes
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Smoothability of compact Alexandrov surfaces with curvature bounded from below

Edit: Addressing Igor's comment I'd like to correct the references I gave. The correct reference for the exact argument I sketch should be the original book by Alexandrov "Intrinsic Geometry of Convex …
Vitali Kapovitch's user avatar
6 votes
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Alexandrov spaces of zero curvature

No, that's too much to ask for. If you take any convex cone in $\mathbb R^n$ (any such cone is both $CAT(0)$ and Alexandrov of $curv\ge 0$) then the tangent space at the origin is just the cone itself …
Vitali Kapovitch's user avatar
3 votes

Isometric classification of 1-dimensional Alexandrov spaces

I don't think anyone bothered to write a formal proof down but this is indeed quite easy. Tangent spaces (which are always metric cones) can only be lines or half lines. From this it's immediate that …
Vitali Kapovitch's user avatar
1 vote

gradient curve $\gamma$ defined on $(-T,0]$, can't be extended from $\gamma(-T)$?

The answer depends on your definition of semi-concave functions. If you only require them to be semi-concave on geodesics then an obvious example is given by $X$ equal to the closed unit ball in $\mat …
Vitali Kapovitch's user avatar