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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× 10

2knots× 14

3manifolds× 216 
4manifold

4manifolds× 83  abcconjecture× 14  abeliancategories× 59 
abeliangroups× 64  abelianschemes× 26 
abelianvarieties× 339
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× 212 
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× 2155
Commutative rings, modules, ideals, homological algebra, computational aspects, invariant theory, connections to algebraic geometry and combinatorics.

accessiblecategories× 6 
acyclicorientations× 2

adamsoperations× 2

additivecombinatorics× 191
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× 9

adeles× 16 
adjacencymatrices× 3

adjacencymatrix

adjointfunctors× 64  admissiblerepresentation× 2  advice× 101  affinegeometry× 10 
affinegrassmannian× 15 
ag.algebraicgeometry× 8982
Algebraic varieties, stacks, sheaves, schemes, moduli spaces, complex geometry, quantum cohomology.

ainfinityalgebras× 35
For questions about $A_\infty$algebras as introduced by Stasheff in 1963 and related structures.

albertalgebras× 2

alexandrovgeometry× 62
the study of the geometry of Alexandrov spaces, which are non smooth analogues of Riemannian manifolds with curvature bounded from below.

algebraiccombinatorics× 58  algebraiccomplexity× 4  algebraiccurves× 402 