tea.mathoverflow.net - Discussion Feed (Continuous bijective way...) Sun, 04 Nov 2018 23:22:45 -0800 http://mathoverflow.tqft.net/ Lussumo Vanilla 1.1.9 & Feed Publisher Noah Stein comments on "Continuous bijective way..." (12347) http://mathoverflow.tqft.net/discussion/877/continuous-bijective-way/?Focus=12347#Comment_12347 http://mathoverflow.tqft.net/discussion/877/continuous-bijective-way/?Focus=12347#Comment_12347 Sun, 02 Jan 2011 17:06:27 -0800 Noah Stein Anton Geraschenko comments on "Continuous bijective way..." (12343) http://mathoverflow.tqft.net/discussion/877/continuous-bijective-way/?Focus=12343#Comment_12343 http://mathoverflow.tqft.net/discussion/877/continuous-bijective-way/?Focus=12343#Comment_12343 Sun, 02 Jan 2011 15:56:26 -0800 Anton Geraschenko 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.

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Andres Caicedo comments on "Continuous bijective way..." (12337) http://mathoverflow.tqft.net/discussion/877/continuous-bijective-way/?Focus=12337#Comment_12337 http://mathoverflow.tqft.net/discussion/877/continuous-bijective-way/?Focus=12337#Comment_12337 Sun, 02 Jan 2011 14:58:39 -0800 Andres Caicedo
I'm opening this thread in case somebody wants to weigh in opinions one way or the other. ]]>