20 Answers votes newest views recent 17Topologically contractible algebraic varieties 13Homotopy groups of Lie groups 8Which R-algebras are the group ring of some group over a ring R? 6What are the implications of torsion in H^2 for geometric quantization? 6Which commutative groups are the group of units of some field? 5Logic comment in Mumford’s Red Book 4D-modules supported on the nilpotent cone 4Over which schemes can there exist non-trivial G_a bundles? 3Hypercube decomposition of perverse sheaves 3How should I think about correspondences? 3Understanding the unreducedness of a subscheme supported on fixed points 2When is the derived category of representations of a finite poset equivalent to its opposite? 2When are there enough projective sheaves on a space X? 2Equivariant Derived Categories via their properties. 2Making D-modules on affine varieties more explicit 1What are the higher homotopy groups of Spec Z ? 1(∞, 1)-categorical description of equivariant homotopy theory 1Thom first isotopy lemma in o-minimal structures 1Is there a theory of differential equations for smooth correspondences? 0Do convolution and multiplication satisfy any nontrivial algebraic identities?