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Jul 13, 2018 at 16:33 comment added Iosif Pinelis @AlexM. : Andreas Blass stated the point of the example exactly right.
Jul 13, 2018 at 13:03 comment added Andreas Blass @AlexM. I think the point of the example is that we have equality of the two curvatures at points with the same $x$-coordinate, not at points the same distance along the curves.
Jul 13, 2018 at 12:42 comment added Alex M. @IosifPinelis: "we would have that the length of the graph of a smooth enough function $f$ over $[a,b]$ would be determined by the curvature of the graph and the values of $f$ at $a,b$." But don't we have this? It seems to me that this is an immediate consequence of the fundamental theorem of curves: given the curvature and torsion (in this case $0$), there exists a unique curve possessing them, modulo isometries (theorem 2.1.7). In particular, its length will be unambiguously determined by the curvature function alone.
Jul 12, 2018 at 23:23 comment added Đào Thanh Oai Your answer is very nice.
Jul 12, 2018 at 23:02 vote accept Đào Thanh Oai
Jul 12, 2018 at 22:21 history edited Iosif Pinelis CC BY-SA 4.0
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Jul 12, 2018 at 22:04 history edited Iosif Pinelis CC BY-SA 4.0
added 4 characters in body
Jul 12, 2018 at 21:57 history answered Iosif Pinelis CC BY-SA 4.0