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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× 14  2knots× 15  3manifolds× 256 
4manifold

4manifolds× 96  abcconjecture× 17  abeliancategories× 69 
abeliangroups× 71  abelianschemes× 27 
abelianvarieties× 377
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× 31 
abstractalgebra× 271 
abstractnonsense× 10
Arguments working entirely at a high level of abstraction, particularly categorytheoretic arguments.

abstractpolytopes× 6
a poset satisfying a list of properties shared by face lattices of polytopes.

ac.commutativealgebra× 2481
Commutative rings, modules, ideals, homological algebra, computational aspects, invariant theory, connections to algebraic geometry and combinatorics.

accessiblecategories× 8 
acyclicorientations× 2

adamsoperations× 3 
additivecombinatorics× 252
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× 11  adeles× 23  adjacencymatrices× 5 
adjacencymatrix

adjointfunctors× 71  admissiblerepresentation× 6  advice× 75  affinegeometry× 18 
affinegrassmannian× 21 
ag.algebraicgeometry× 10292
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× 98
Alexandrov geometry studies non smooth analogues of Riemannian manifolds with curvature bounded from below or above. It includes spaces with curvature bounded below (briefly $\mathrm{CBB}[\kappa]$) an…

algebraiccombinatorics× 80  algebraiccomplexity× 9  algebraiccurves× 489 