Friday, 25 April 2008

at.algebraic topology - How should I visualise RP^n?

I find it easiest to get mathbbRPn from Sn with two cells in every dimension. That is to say, you start with two points for S0, attach two one-cells oriented opposite ways for S1, attach one 2-cell on top and one on the bottom for S2, and so on until you have Sn. Then the antipodal map actually switches the two cells in each dimension, so it defines a free mathbbZ/2 action on Sn. If you quotient out by that action, then you get a cell structure on mathbbRPn with one cell in each dimension, and you have a universal double cover (n>1) for it.



I like this construction for a couple of reasons. First, it displays the (co)boundary maps in cellular (co)homology: the two k-cells share the same boundary (up to sign (1)k) which is the two k1-cells. Because it's a covering space you can sort of see how after quotienting you get one k-cell whose boundary folds twice onto the k1-cell. So that also shows you why the maps alternate between the 0 map and the multiplication by 2, or are all the 0 map in mathbbZ/2 coefficients.



Also, this generalizes to a construction of Sinfty and thus mathbbRPinfty, with mathbbRPn inside it just by taking the n-skeleton. It looks like you could modify Qiaochu's definition to account for the infinite case by taking a k-cell to be the points that vanish after the first k+1 dimensions and are scaled to 1 in the k+1st.

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