# Help:Categories

This pages lists policies and manuals regarding the use of categories, an organizational feature that makes locating $\mathsf{Pr} \infty \mathsf{fWiki}$ pages easier.

## Assigning a Category

Each page should reference at least one category, referenced by placing [[Category:CategoryName]] in the page code. For organizational purposes this is to be added at the very bottom of the page.

So for example, a proof about the properties of sets could go under the category "Set Theory", which is entered as [[Category:Set Theory]].

A category should not be added when a more specific category applies. For example, Group theory can be omitted when Cyclic Groups is suitable.

### Keeping categories small

There is a tendency to try to limit the number of members of a category. Specifically, pages that would fit into a subcategory are typically only placed in the subcategory, so as to keep the main category small. This applied to both Proofs and Definitions. Some remarks are in order:

• Main theorems in a certain field or about a concept are often added to the corresponding category, even if they also belong to a subcategory. This is useful, as it gives an overview of the main theorems in that field, without smaller results appearing in the list.
• For definition categories, the definition of a concept for which there is a subcategory, is often placed in both the subcategory and the main category.

## Creating a new category

Sometimes there arises a need for a new category. For example, a contributor could start working on an as-of-yet uncovered field of mathematics. Alternatively, it can be necessary to split a category into several more specific subcategories if it gets too large. As always with creating a new page, make sure the category does not already exist or has a different name that the one you had in mind.

Several conventions on the creation of categories are in place. In due time, these will be explained here, as well as some useful constructs that make managing and creating categories easier. In the mean time, contact one of the ProofWiki sysops for help.

### Naming conventions

The general naming conventions apply to categories as much as they do to other pages. Categories are simply pages that reside in the "Category" namespace, meaning that their title starts with Category:.

#### Capitalization

The guidelines for capitalization are the same as for other pages. See Help:Page Naming.

#### Plural versus Singular

Plural when the object being categorized is a collective term for objects; e.g. Category:Limits, Category:Axioms, Category:Cycloids, Category:Cyclotomic Polynomials.

Singular when there is only one such object; e.g. Category:Riemann Zeta Function, Category:Group Theory, Category:Algebra (as opposed to Category:Algebras, which is used for actual objects.)

Things like Category:Asymptotic Notation

### The category hierarchy

Each new category page also needs to be assigned to a category itself. This is done like on normal pages, by adding [[Category:SuperCategory]] to the category page. The uppermost category in the hierarchy is Category:Categories, being the only one that does not belong to a supercategory.

For categories of proofs, the universal category is that of Proofs. If a category is a subcategory of something else, then it suffices to add that something else as a category; the MediaWiki architecture will ensure it can be reached from Proofs.

## Special Categories

### Example Categories

The way in which example categories are named has recently been changed. Consequently, not all example categories currently follow this new convention. More info on how to add example categories will be added in due time.

### Subpages

In order to avoid duplicate entries in page lists, a recent practice is to create a category for each page with many subpages. For example, when a theorem has many proofs, they are all placed in a category, which gets the same name as the main page on which the subpages are transcluded.