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: 
highercategorytheory× 451 
kt.ktheoryhomology× 442
Algebraic and topological Ktheory, relations with topology, commutative algebra, and operator algebras

computabilitytheory× 441
computable sets and functions, Turing degrees, c.e. degrees, models of computability, primitive recursion, oracle computation, models of computability, decision problems, undecidability, Turing jump, …

convexpolytopes× 439
the convex hulls of a finite set of points in Euclidean spaces. They have rich combinatorial, arithmetic, and metrical theory, and are related to toric varieties and to linear pr…

forcing× 434
a method first used to prove the continuum hypothesis is independent of the classical axioms of set theory

terminology× 423
Questions of the kind "What's the name for a X that satisfies property Y?"

ergodictheory× 422
Dynamical systems on measure spaces, invariant measures, ergodic averages, mixing properties.

tagremoved× 412
to be used only when retagging highly(!) offtopic questions where none of the actual tags would make sense; all actual tags the questioner has used are removed and something is needed to…

sp.spectraltheory× 410
Schrodinger operators, operators on manifolds, general differential operators, numerical studies, integral operators, discrete models, resonances, nonselfadjoint operators, random operators/matrices…

integration× 402
Questions related to various forms of integration including the Riemann integral, Lebesgue integral, Riemann–Stieltjes integral, double integrals, line integrals, contour integrals, surface integrals,…

abelianvarieties× 391
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. …

hyperbolicgeometry× 386 
sheaftheory× 381 
computerscience× 380
For question borderline with, or having application to, computer science. Consider also posting http://cs.stackexchange.com/ or http://cstheory.stackexchange.com/ instead of here, if appropriate.

geometricgrouptheory× 377
Large scale properties of groups; growth functions; Dehn functions; small cancellation properties; hyperbolicity and CAT(0); actions and representations; combinatorial group theory; presentations

simplicialstuff× 373 
galoistheory× 372  groupcohomology× 370 
convexgeometry× 355
A branch of geometry dealing with convex sets and functions. Polytopes, convex bodies, discrete geometry, linear programming, antimatroids, ...

sobolevspaces× 355
a vector space of functions equipped with a norm that is a combination of Lpnorms of the function itself and its derivatives up to a given order.

openproblem× 352
equivalent to a known open problem, then the openproblem tag is added. After that, the question essentially becomes, "What is known about this problem? What are some…

specialfunctions× 350
Many special functions appear as solutions of differential equations or integrals of elementary functions. Most special functions have relationships with representation theory of Lie groups.

examples× 349 
lattices× 343
used in number theory. (Not to be confused with lattice theory or lattices as used in physics!)

diophantineequations× 339
polynomial equations $F=0$, or systems of polynomial equations $F_1=\ldots=F_k=0$, where $F,F_1,\ldots,F_k$ are polynomials in either $\mathbb{Z}[X_1,\ldots,X_n]$ of $\mathbb…

stochasticcalculus× 325
Stochastic calculus provides a consistent theory of integration for stochastic processes and is used to model random systems. Its applications range from statistical physics to quantitative finance.

modelcategories× 321 
operatortheory× 321
Spectrum, resolvent, numerical range, functional calculus, operator semigroups. Special classes of operators: compact, Fredholm, dissipative, differential, integral, pseudodifferential, etc.

cstaralgebras× 319
a complex Banach algebra together with an isometric antilinear involution satisfying (a b)* = b* a* and the C*identity ‖a* a‖ = ‖a‖². Related tags: [tag:banachalgebras], [tag:vonne…

riemannsurfaces× 317  convexity× 315  deformationtheory× 315 
asymptotics× 310  largecardinals× 309 
ellipticpde× 305
Questions about partial differential equations of elliptic type. Often used in combination with the toplevel tag ap.analysisofpdes.

automorphicforms× 302
a wellbehaved function from a topological group G to the complex numbers (or complex vector space) which is invariant under the action of a discrete subgroup \Gamma \subset G o…
