Main Question or Discussion Point
so first let's take RP^2. I have a little trouble grasping why we can put a subspace topology on it. So RP^2 is the set of all lines through the origin in R^3. So if we take some subset S of RP^2 and the if set of points in R^3 which is the union of these lines in S is open then we can say we take the intersection of S and the open subset of R^3 and get a subspace topology. However, apparently you can only get the indiscrete topology? the reason i read for this was because the union would contain 0 but you can't have an open ball around 0. Let's say you take a single line as your subset S. you can't put an open ball around any of the points right? you won't necessarily have some delta in the x, y, z directions? how come the argument says you can't put an open ball around 0, but you can put it around the other points?