A tag is a keyword or label that categorizes your question with other, similar questions. Using the right tags makes it easier for others to find and answer your question.
Type to find tags: 
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× 17  2knots× 15  3manifolds× 272 
4manifold

4manifolds× 100  abcconjecture× 20  abeliancategories× 73 
abeliangroups× 85  abelianschemes× 29 
abelianvarieties× 399
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× 36 
abstractalgebra× 293
Do NOT use this tag. Instead you could consider gr.grouptheory, ac.commutativealgebra, ra.ringsandalgebras or various more specific tags.

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× 2632
Commutative rings, modules, ideals, homological algebra, computational aspects, invariant theory, connections to algebraic geometry and combinatorics.

accessiblecategories× 8 
acyclicorientations× 2

adamsoperations× 3 
additivecombinatorics× 267
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× 73  admissiblerepresentation× 5  advice× 75  affinegeometry× 22 
affinegrassmannian× 21 
ag.algebraicgeometry× 10867
Algebraic varieties, stacks, sheaves, schemes, moduli spaces, complex geometry, quantum cohomology.

ainfinityalgebras× 41
For questions about $A_\infty$algebras as introduced by Stasheff in 1963 and related structures.

albertalgebras× 2
a 27dimensional exceptional Jordan algebra.

alexandrovgeometry× 104
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× 85  algebraiccomplexity× 10 
algebraiccurves× 507
An algebraic curve or plane algebraic curve is the set of points on the Euclidean plane whose coordinates are zeros of some polynomial in two variables.
