Is the set of solution to $x_0^2+x_1^2-x_2^2=0$ homeomorphic to the set of solution to $x_0^2-x_1^2=0$ in $ \mathbb{P}^2 (\mathbb{R})$? I think one can show that the first set is homeomorphic to $S^1$, and that by removing the point $[0,0,1]$ from the second it is no more connected, hence they aren't homeomorphic. However I would like to learn other solutions or have a confirmation.
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2$\begingroup$ Yes, your solution is completely correct. $\endgroup$– user64687Jan 14, 2015 at 15:47