Wikipedia:WikiProject Mathematics/Conventions

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This page is to collect up various mathematics conventions used in Wikipedia. Where there is not complete consensus in the mathematical literature, we want to make consistent choices across articles.

It should not be assumed that the readers of articles are familiar with the conventions listed here. Therefore, when an article could be misunderstood if interpreted according to a convention that is reasonably current in the mathematical literature, the article should make a note of that fact in situ.

[edit] Proposals for new conventions

If you wish to propose a convention that is not already reasonably standard here, add to the top of the list below, and start a discussion on the talk page. Leave the discussion open for at least two weeks, before implementing any large-scale edits based on the convention. Don't assume everyone concerned is watching this page.

(Terminology) The term "bipyramid" should be preferred over "dipyramid".
(Notational) Infinity notation: $+\infty$ should be used for positive infinity; $-\infty$ should be used for negative infinity; $\infty$ should be reserved for unsigned infinity, as in the real projective line or the Riemann sphere.
(Terminology) An algebraic variety is assumed to be an irreducible algebraic set.

[edit] Terminology conventions

These should be regarded as mandatory; any article needing another convention should say so clearly

Compact spaces are not assumed Hausdorff (contra Bourbaki, who uses quasi-compact for our notion of compactness).
See completely Hausdorff space.
A ring is associative and unital (Exception: rings of operators, such as * algebras, B* algebras, C* algebras need not be unital).
A local ring is not assumed noetherian (contra Zariski).
In scheme theory, the older terminology of preschemes is not used (but scheme (mathematics) and separated scheme).
For Clifford algebras use v² = +Q(v).
The weight k of a modular form follows a currently-accepted (Serre) convention (so f(−1/τ) = τ^kf(τ) for level k), and q = e^2πiτ.
Elliptic functions are written in ω = half-period style.
Directed sets are preordered sets with finite joins, not partial orders as in, e.g., Kelley (General Topology; ISBN 0-387-90125-6).

[edit] Notational conventions

These are advisory. Articles not adopting them may confuse the reader, and therefore should make an effort to be clear about the notation in use

The abstract cyclic group of order n, when written additively, has notation Z_n, or in contexts where there may be confusion with p-adic integers, Z/nZ; when written multiplicatively, e.g. as roots of unity, C_n is used (this does not affect the notation of isometry groups called C_n ).
The abstract dihedral group of order 2n has notation Dih_n (but note that the notation D_2n is the standard one in finite group theory outside wikipedia); this does not affect the notation of isometry groups (one type in 2D and one in 3D, both of order 2n, are called D_n )
Bernoulli numbers are denoted by B_n, and are zero for n odd and greater than 1.
In category theory, write Hom-sets, or morphisms from A to B, as Hom(A,B) rather than Mor(A,B) (and with the implied convention that the category is not a small category unless that is said).
The semidirect product of groups K and Q should be written K ×_φ Q or Q ×_φ K where K is the normal subgroup and φ : Q → Aut(K) is the homomorphism defining the product. The semidirect product may also be written K ⋊ Q or Q ⋉ K (with the bar on the side of the non-normal subgroup) with or without the φ.
- The context should clearly state that this is a semidirect product and should state which group is normal.
- The bar notation is discouraged for reasons outlined on the talk page.
- If the bar notation is used it should be entered as {{unicode|⋉}} (⋉) or {{unicode|⋊}} (⋊) for maximum portability.
Subset is denoted by $\subseteq$ , proper subset by $\subsetneq$ . The symbol $\subset$ may be used if the meaning is clear from context, or if it is not important whether it is interpreted as subset or as proper subset (for example, $A\subset B$ might be given as the hypothesis of a theorem whose conclusion is obvious in the case that $A = B$ ). All other uses of the $\subset$ symbol should be explicitly explained in the text.