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Aug 29, 2017 at 16:44 comment added Robert Bryant It might be worth mentioning that it is known that any (smooth) metric on a compact surface can be smoothly isometrically embedded into $\mathbb{R}^5$. (Anton's method below constructs an explicit ellipsoidal metric embedding of $\mathbb{RP}^2$ into $\mathbb{R}^6$.) Meanwhile, it appears still to be unknown whether there is any metric on $\mathbb{RP}^2$ with positive Gauss curvature that can be smoothly isometrically embedded into $\mathbb{R}^4$. Cf., the discussion in M. Gromov's, Partial Differential Relations, Section 3.2.4.
Aug 28, 2017 at 21:11 answer added Anton Petrunin timeline score: 6
Aug 24, 2017 at 6:25 history asked Min-Oo CC BY-SA 3.0