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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 $\mathrm{CAT}[\kappa]$).

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Is $\partial X$ a sphere for $X$ a complete CAT$(0)$ space?

For a piecewise Euclidean or piecewise hyperbolic metric on a PL manifold, the answer is YES. This is proved (p348 in published version) by M. Davis and T. Januszkiewicz in M. Davis, T.Januszkiewicz, …
YCor's user avatar
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9 votes

Geodesics on convex hypersufaces

The answer to the question as stated is NO. Indeed, consider (the boundary of) a tetrahedron (or any convex polyhedron). Since the cone angles at the vertices are smaller than $2\pi,$ no geodesic can …
Igor Rivin's user avatar
  • 96.4k
0 votes

Embedding expanders in CAT(0) spaces

See the recent preprint http://arxiv.org/abs/1306.5434 (Mendel and Naor).
Igor Rivin's user avatar
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2 votes

Source for: Geodesics in CAT(0) spaces

The canonical reference is Bridson and Haefliger, Metric Spaces of non-positive curvature
Igor Rivin's user avatar
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6 votes

Connecting Lemma in the Alexandrov's existence theorem.

I am not aware of an elementary argument along these lines. A high-powered argument to show a priori connectedness uses Teichmuller theory (the space of polyhedral metrics fibers over teichmuller/modu …
Igor Rivin's user avatar
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7 votes

Examples of CAT(0)-groups

The answer is yes, since $M$ is a $CAT(0)$ space, and the group is quasi-isometric to it. See (for example) Jim Cannon's article in Bedford Keane Series: The theory of negatively curved spaces and gr …
Igor Rivin's user avatar
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