how to prove that this sequence not converges uniformly in all the real line? I don“t know how to do this problem :S , in fact I think that it converges uniformly in "all the real line" The sequence is $ S_n \left( x \right) = \sum\limits_{k = 1}^n {\frac{{x^k }} {{k!}}} $
how to prove that this sequence not converges uniformly?
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real-analysis
1 Answers
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The limit for $n\to\infty$ is easily seen to be $e^x-1$. However since $S_n(x)$ is a polynomial, it will always take on absolutely-large values for sufficiently large negative $x$. But for all such $x$, $e^x-1$ is close to $-1$.
Even without knowing what the limit is, however, the $S_n$'s are polynomials of alternating odd and even degrees. Therefore for any $N$ there is an $x$ that is so negative that $S_N(x)$ has a different sign from $S_{N+1}(x)$ and both are large.