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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× 5

2knots× 14  3manifolds× 190 
4manifold

4manifolds× 69  abcconjecture× 10  abeliancategories× 49 
abeliangroups× 50  abelianschemes× 22 
abelianvarieties× 286
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× 25 
abstractalgebra× 181 
abstractnonsense× 7
Arguments working entirely at a high level of abstraction, particularly categorytheoretic arguments.

abstractpolytopes× 3 
ac.commutativealgebra× 1915
Commutative rings, modules, ideals, homological algebra, computational aspects, invariant theory, connections to algebraic geometry and combinatorics.

accessiblecategories× 6 
acyclicorientations× 2

adamsoperations× 2

additivecombinatorics× 150
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× 13  adjacencymatrices× 3 
adjacencymatrix

adjointfunctors× 52 
admissiblerepresentation× 2

advice× 99 
affinegeometry× 6

affinegrassmannian× 9 
ag.algebraicgeometry× 7746
Algebraic varieties, stacks, sheaves, schemes, moduli spaces, complex geometry, quantum cohomology.

ainfinityalgebras× 34 
albertalgebras× 2

alexandrovgeometry× 53
the study of the geometry of Alexandrov spaces, which are non smooth analogues of Riemannian manifolds with curvature bounded from below.

algebraiccombinatorics× 42 
algebraiccomplexity× 3

algebraiccurves× 328 