# Pointed space

{{ safesubst:#invoke:Unsubst||\$N=Refimprove |date=__DATE__ |\$B= {{#invoke:Message box|ambox}} }} Template:Cleanup In mathematics, a pointed space is a topological space X with a distinguished basepoint x0 in X. Maps of pointed spaces (based maps) are continuous maps preserving basepoints, i.e. a continuous map f : XY such that f(x0) = y0. This is usually denoted

f : (X, x0) → (Y, y0).

Pointed spaces are important in algebraic topology, particularly in homotopy theory, where many constructions, such as the fundamental group, depend on a choice of basepoint.

The pointed set concept is less important; it is anyway the case of a pointed discrete space.

## Category of pointed spaces

The class of all pointed spaces forms a category Top with basepoint preserving continuous maps as morphisms. Another way to think about this category is as the comma category, ({•} ↓ Top) where {•} is any one point space and Top is the category of topological spaces. (This is also called a coslice category denoted {•}/Top.) Objects in this category are continuous maps {•} → X. Such morphisms can be thought of as picking out a basepoint in X. Morphisms in ({•} ↓ Top) are morphisms in Top for which the following diagram commutes:

It is easy to see that commutativity of the diagram is equivalent to the condition that f preserves basepoints.

As a pointed space {•} is a zero object in Top while it is only a terminal object in Top.

There is a forgetful functor TopTop which "forgets" which point is the basepoint. This functor has a left adjoint which assigns to each topological space X the disjoint union of X and a one point space {•} whose single element is taken to be the basepoint.

## References

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