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: 
etalecohomology× 250  ordertheory× 249  galoisrepresentations× 247  topologicalgroups× 246 
finitefields× 245  asymptotics× 244 
operatortheory× 243
Spectrum, resolvent, numerical range, functional calculus, operator semigroups. Special classes of operators: compact, Fredholm, dissipative, differential, integral, pseudodifferential, etc.

axiomofchoice× 242 
randommatrices× 241
Statistics of spectral properties of matrixvalued random variables.

bigpicture× 239
Questions designed to get an overview of a specific subject or body of results or to understand the relations among similar definitions, techniques or concepts appearing in different subfields of mat…

ellipticpde× 238
Questions about partial differential equations of elliptic type. Often used in combination with the toplevel tag ap.analysisofpdes.

fields× 236
Fields as algebraic objects. For vector and tensor fields, use eg. [dg.differentialgeometry]. For physical fields, use eg. [mp.mathematicalphysics] or [quantumfieldtheory].

abstractalgebra× 236  singularitytheory× 231  3manifolds× 230 
derivedcategory× 228
For questions about the derived categories of various abelian categories and questions regarding the derived category construction itself.

additivecombinatorics× 222
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,…

quadraticforms× 222  modules× 220 
noncommutativegeometry× 220
Noncommutative geometry in the sense of Connes and beyond: noncommutative algebras viewed as functions on a noncommutative space.

quantumgroups× 219
Questions about algebraic structures known as quantum groups, and their categories of representations. Quasitriangular Hopf algebras and their Drinfel'd twists, triangular Hopf algebras, $C^\star$ qua…

noncommutativealgebra× 218  vonneumannalgebras× 217  linearprogramming× 215 
intuition× 215
Questions asking for the intuition behind some definition, conjecture, proof etc. In other words, questions designed to improve or to acquire understanding on a conceptual or intuitive level, as oppos…

projectivegeometry× 214 
matrixanalysis× 213
The study of the properties of real and complex matrices that are more close to analysis and operator theory. For instance: the properties of positive definite matrices, matrix inequalities, perturbat…

mathematicseducation× 213
For questions in Mathematics Education as a scientific discipline. For more handson questions on teaching Mathematics, please use the tag teaching. There is also a Stack Exchange community http://mat…

topostheory× 213 
descriptivesettheory× 211
the study of definable subsets of Polish spaces, where definable is taken to mean from the Borel or projective hierarchies. Other topics include infinite games and determinac…

hopfalgebras× 210
a vector space $H$ over a field $k$ endowed with an associative product $\times:H\otimes_k H\to H$ and a coassociative coproduct $\Delta:H\to H\otimes_k H$ which is a morphism of alg…

commutativerings× 209 
algebraicktheory× 205  combinatorialgeometry× 195  markovchains× 194  approximationtheory× 192 