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  1.  
    (Not having thought deeply about it) I think there is a potentially interesting question here, and have voted to reopen: http://mathoverflow.net/questions/50932/continuous-bijective-way-of-representing-a-line-on-a-plane-closed

    I'm opening this thread in case somebody wants to weigh in opinions one way or the other.
  2.  

    I vote to reopen it (meaning that I'll vote to reopen once some other people do). The original question (where "bijective" read "1-1") was awful because of the (sort of sensible) confusion that "injective" = "1-1 or 0-1" and "bijective" = "1-1".

    I think there is no such bijection. A line in the plane is almost the same as a plane through the origin in 3-space (intersecting with the plane at height 1), except there's one plane through the origin that doesn't give you a line (the z=0 plane). So the space of lines in the plane is homeomorphic to $\mathbb{RP}^2$ minus a point: an open mobius strip! So the question is asking if there is a continuous bijection $f:D\to M$ from the open disk $D$ to the open mobius strip $M$.

    Suppose there were such a bijection $f$, then $f$ would induce a continuous bijection of one-point compactifications, $\bar f:\bar D\to \bar M$, which would have to be a homeomorphism (a continuous bijection from a compact space to a hausdorff space is a homeomorphism). But $\bar D=S^2$ and $\bar M = \mathbb{RP}^2$ are not homeomorphic.

    • CommentAuthorNoah Stein
    • CommentTimeJan 2nd 2011 edited
     
    @Anton: Very nice, but I believe a continuous map needs to be proper for the induced function on one-point compactifications is also continuous. So you have shown that there is no proper map of the desired type, but I think maybe a bit more is required to show there is no continuous map at all?