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: 
stablehomotopy× 157  manifolds× 155  mathphilosophy× 154  algebraicsurfaces× 152 
ellipticpde× 152
Questions about partial differential equations of elliptic type. Often used in combination with the toplevel tag ap.analysisofpdes.

matrixanalysis× 150
The study of real and complex matrices and their algebraic and analytical properties, including: eigenvalues and eigenvectors, positive definite matrices, matrix inequalities, invariant subspaces, per…

operatortheory× 150
Spectrum, resolvent, numerical range, functional calculus, operator semigroups. Special classes of operators: compact, Fredholm, dissipative, differential, integral, pseudodifferential, etc.

approximationtheory× 148 
birationalgeometry× 143  monoidalcategories× 140 
langlandsconjectures× 138
Higher reciprocity laws

randomwalk× 138 
computationalgeometry× 138
Using computers to solve geometric problems. Questions with this tag should typically have at least one other tag indicating what sort of geometry is involved, such as ag.algebraicgeometry or mg.metr…

hilbertspaces× 138 
textbookrecommendation× 138
Questions asking for recommendations of textbooks on some subject. It can be helpful to indicate whether the request is for selfstudy, for use in a course one teaches, for use accompanying a course o…

computeralgebra× 137
Using computers to solve algebraic problems. Questions with this tag should typically include at least one other tag indicating what sort of algebraic problem is involved, such as ac.commutativealgeb…

prooftheory× 137
For question in Proof Theory, where "proofs" themselves are the object of mathematical investigation. It is not to be used to request a proof of some result.

semigroups× 136 
books× 132
Questions in which books play a keyrole, such as questions on antique books, ebooks, difference between various editions of a book, etc. For questions asking for recommendations of books on some sub…

riemannzetafunction× 131
the function of one complex variable $s$ defined by the series $\zeta(s) = \sum_{n \geq 1} \frac{1}{n^s}$ when $\operatorname{Re}(s)>1$. It admits a meromorphic continuati…

homology× 129  universalalgebra× 129  zetafunctions× 129 
lfunctions× 127
Questions about generalizations of the Riemann Zeta function of arithmetic interest whose definition relies on meromorphic continuation of special kinds of Dirichlet series, such as Dirichlet Lfuncti…

gm.generalmathematics× 127  euclideangeometry× 126  calculusofvariations× 126  linearprogramming× 122 
career× 121  intersectiontheory× 120  classfieldtheory× 118  groupactions× 117 
permutations× 116
Questions related to permutations, bijections from a finite (or sometimes infinite) set to itself.

convexoptimization× 114  graphcolorings× 111 
differentialoperators× 111
Elliptic, parabolic and hyperbolic operators. Laplace, LaplaceBeltrami, Schrödinger, Dirac. Exterior derivative and Lie derivative operators.
