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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.

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…
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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…
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a space that locally looks like Euclidean 3-dimensional space
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a 4-manifold with a smooth structure. In dimension four, in marked contrast with lower dimensions, topological and smooth manifolds are quite different.
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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. …
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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.
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Arguments working entirely at a high level of abstraction, particularly category-theoretic arguments.
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a poset satisfying a list of properties shared by face lattices of polytopes.
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Commutative rings, modules, ideals, homological algebra, computational aspects, invariant theory, connections to algebraic geometry and combinatorics.
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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. …
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For questions about Adams operations which are cohomology operations for K theory.
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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,…
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for questions on algebraic geometry, including algebraic varieties, stacks, sheaves, schemes, moduli spaces, complex geometry, quantum cohomology.
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For questions about $A_\infty$-algebras as introduced by Stasheff in 1963 and related structures.
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a 27-dimensional exceptional Jordan algebra.