Does anybody know how to go about introducing manifold structure to the set X of all lines in R2 (Real 2D space) and write the charts explicitly????
See the Wikipedia page on the real projective plane, in particular the section on homogeneous coordinates. The essence of the construction is that a line in R^2 is given by a linear equation ax+by+c=0 (but you can multiply a, b and c by any nonzero constant and the equation will still represent the same line). So we can take the set of all lines with c≠0 and represent each line L by an equation ax+by+1=0. This yields a chart in which L is mapped to the point (a,b) in R^2.
What about charts covering lines of the form ax+by=0? Simple: just shift the origin to a point not on the line and then use the same construction as above.
If I used the equation y=mx+c for the equation of a line, then map (m,c) in R2 to (y=mx+c) in R2, then this takes care of all lines that are not vertical. All these lines would be of the form x=d, so I could map d in R to (x=d) in R2. So there would be two charts, mapping to the set of all real lines. would this have the same overall outcome?
You can get around this by taking the second chart to be the map taking (m,c) to the line x=my+c. That map takes care of all lines that are not horizontal. So the two charts together form an atlas for the manifold.
Thankyou very much. so to add manifold structure to the set and explicitly write the charts, I could say the set is frully described by the equations y=mx+c and x=ny+d and the charts writen explictly are f: (m,c) to (y=mx+c) and g: (n,d) to (x=ny+d) with both codomains and domains being R2...????
If you want to complete the specification, then the transition map from f to g is the map with domain (because the line is the same as the line ).