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Mar 20, 2015 at 19:30 comment added Igor Rivin Ah, did not notice the 1-1 hypothesis...
Mar 20, 2015 at 18:25 comment added Ramiro de la Vega Indeed, the interesting case is when the function is not one to one (space-filling curves). For a one-to-one function from $\mathbb{R}$ to $\mathbb{R}^2$, the image of each $[-n, n]$ must be a convex set homeomorphic to $[0,1]$, hence a line segment.
Mar 20, 2015 at 18:12 comment added Joonas Ilmavirta The theorem reads "Let $V$ and $W$ be real vector spaces. Any one-to-one mapping $f:V\to W$ which preserves convexity is collinearity-preserving." The OP did not assume the function to be one-to-one, so this does not fully answer the question.
Mar 20, 2015 at 18:03 history answered Igor Rivin CC BY-SA 3.0