The TQC Lemma
Note: This is an edited version of the original post. Thanks to Prof. Rolf Schnieder for alerting me that the Spherical Hadwiger conjecture remains open. Sometime in late 2012 or early 2013, Joel Tropp, Martin Lotz, Dennis Amelunxen, and I were all discussing some integral-geometric problem or another, and the topic of projections of convex cones came up. One of us made the observation that taking the linear image of a subspace in general position yields one of two results: ...