# Tags

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.

0-1-matrices

Questions involving results for the special case of matrices over the integers, reals, or complex numbers in which all coefficients are 0 or 1 (e.g. permutation matrices and adjacency matrices of grap…

20-questions× 18

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

4-manifolds× 147

a 4-manifold with a smooth structure. In dimension four, in marked contrast with lower dimensions, topological and smooth manifolds are quite different.

abelian-varieties× 550

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. …

abstract-algebra× 314

Deprecated; do NOT use this tag. Instead you could consider gr.group-theory, ac.commutative-algebra, ra.rings-and-algebras, universal-algebra, or various more specific tags.

abstract-nonsense× 15

Arguments working entirely at a high level of abstraction, particularly category-theoretic arguments.

abstract-polytopes× 13

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

ac.commutative-algebra× 3652

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

acyclic-orientations× 2

An acyclic orientation of an undirected graph is an assignment of a direction to each edge (an orientation) that does not form any directed cycle and therefore makes it into a directed acyclic graph. …

adams-operations× 4

For questions about Adams operations which are cohomology operations for K theory.

additive-combinatorics× 400

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,…

ag.algebraic-geometry× 14611

for questions on algebraic geometry, including algebraic varieties, stacks, sheaves, schemes, moduli spaces, complex geometry, quantum cohomology.

a-infinity-algebras× 67

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