Pointed set

In mathematics, a pointed set (also based set or rooted set) is an ordered pair $(X, x_0)$ where $X$ is a set and $x_0$ is an element of $X$ called the base point, also spelled basepoint.

Maps between pointed sets $(X, x_0)$ and $(Y, y_0)$ (called based maps, pointed maps, or point-preserving maps) are functions from $X$ to $Y$ that map one basepoint to another, i.e. a map $f : X \to Y$ such that $f(x_0) = y_0$. This is usually denoted

$f : (X, x_0) \to (Y, y_0)$.

Pointed sets may be regarded as a rather simple algebraic structure. In the sense of universal algebra, they are structures with a single nullary operation which picks out the basepoint.

The class of all pointed sets together with the class of all based maps form a category. In this category the pointed singleton set $(\{a\}, a)$ is an initial object and a terminal object, i.e. a zero object. There is a faithful functor from usual sets to pointed sets, but it is not full and these categories are not equivalent. In particular, the empty set is not a pointed set, for it has no element that can be chosen as base point.

The category of pointed sets and based maps is equivalent to but not isomorphic with the category of sets and partial functions. One textbook notes that "This formal completion of sets and partial maps by adding “improper,” “infinite” elements was reinvented many times, in particular, in topology (one-point compactification) and in theoretical computer science."

The category of pointed sets and pointed maps is isomorphic to the co-slice category $\mathbf{1} \downarrow \mathbf{Set}$, where $\mathbf{1}$ is a singleton set.

The category of pointed sets and pointed maps has both products and co-products, but it is not a distributive category.

Many algebraic structures are pointed sets in a rather trivial way. For example, groups are pointed sets by choosing the identity element as the basepoint, so that group homomorphisms are point-preserving maps. This observation can be restated in category theoretic terms as the existence of a forgetful functor from groups to pointed sets.

A pointed set may be seen as a pointed space under the discrete topology or as a vector space over the field with one element.

As "rooted set" the notion naturally appears in the study of antimatroids and transportation polytopes.

See also

• Accessible pointed graph

External links

• http://mathoverflow.net/questions/22036/pullbacks-in-category-of-sets-and-partial-functions
• Pointed Set at PlanetMath
• http://ncatlab.org/nlab/show/pointed+object