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20questions× 18
In the earliest days of MathOverflow, there was a '20 questions' seminar (see <http://sbseminar.wordpress.com/category/20questions/>) run by graduate students at Berkeley. Many questions from the sem…

2categories× 13 
2knots× 14

3manifolds× 234 
4manifold

4manifolds× 93  abcconjecture× 14  abeliancategories× 66 
abeliangroups× 68  abelianschemes× 27 
abelianvarieties× 357
projective algebraic varieties endowed with an Abelian group structure. Over the complex numbers, they can be described as quotients of a vector space by a lattice of full rank. …

absolutegaloisgroup× 27 
abstractalgebra× 255 
abstractnonsense× 10
Arguments working entirely at a high level of abstraction, particularly categorytheoretic arguments.

abstractpolytopes× 5
a poset satisfying a list of properties shared by face lattices of polytopes.

ac.commutativealgebra× 2383
Commutative rings, modules, ideals, homological algebra, computational aspects, invariant theory, connections to algebraic geometry and combinatorics.

accessiblecategories× 6

acyclicorientations× 2

adamsoperations× 2

additivecombinatorics× 231
Questions on the subject additive combinatorics, also known as arithmetic combinatorics, such as questions on: additive bases, sum sets, inverse sum set theorems, sets with small doubling, Sidon sets,…

adeclassifications× 10  adeles× 19 
adjacencymatrices× 3

adjacencymatrix

adjointfunctors× 67  admissiblerepresentation× 5  advice× 71  affinegeometry× 14 
affinegrassmannian× 18 
ag.algebraicgeometry× 9836
Algebraic varieties, stacks, sheaves, schemes, moduli spaces, complex geometry, quantum cohomology.

ainfinityalgebras× 38
For questions about $A_\infty$algebras as introduced by Stasheff in 1963 and related structures.

albertalgebras× 2

alexandrovgeometry× 81
the study of the geometry of Alexandrov spaces, which are non smooth analogues of Riemannian manifolds with curvature bounded from below.

algebraiccombinatorics× 71  algebraiccomplexity× 8  algebraiccurves× 463 