# CW structure of real projective space

This article discusses a common choice of CW structure for real projective space , i.e., a CW-complex having this as its underlying topological space.

## Description of cells and attaching maps

There is one cell in dimension . Thus, there is a total of different cells. Note that:

- The -skeleton is homeomorphic to , and in fact, the CW structure induced on this skeleton is the same as the CW structure we would have chosen for independently
- The attaching map at stage is the map arising from the fiber bundle of sphere over projective space (complex case) .

A more concrete way of interpreting these cells and attaching maps is as follows. Choose a basis for . is the space of lines through the origin in . The -skeleton is the subspace comprising those lines that lie inside the subspace spanned by the first basis vectors. Each time we add a new cell, we are allowing directions that lie in the span of one more basis vector.

## Cellular chain complex and cellular homology

Any CW structure on a topological space provides a cellular filtration relative to the empty space. The corresponding cellular chain complex is described below. By excision, the cellular chain group is where is the number of -cells.

For the case of , since there is one cell in dimension for , the cellular chain groups are in dimensions for are elsewhere.

The cellular chain complex thus looks like:

The boundary maps are as follows: the map for even, is the doubling map. For odd, the map is the zero map.

This gives the homology groups as described in homology of real projective space.