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Oct 6, 2012 at 2:10 comment added user21349 This example can be done straightforwardly without limits, using infinitesimals (either informally or in NSA).
Apr 3, 2011 at 23:08 comment added Toby Bartels For the more general example, one should also ask that $f$ is not differentiable at $0$. (By varying such $f$, we can get any derivative at all, or none. Taking $f(x) = x \sin(1/x)$, extended continuously, gives a very interesting example that's come up elsewhere on this page.)
Sep 27, 2010 at 17:12 comment added Igor Belegradek If you want example with nonzero derivative consider $x|x|+x$.
Sep 27, 2010 at 15:33 history answered Igor Belegradek CC BY-SA 2.5