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I have two asymptotic notation questions.

Determine whether or not $\frac{1}{\ln(n)}=O(\frac{1}{n})$

My answer I put the statement is not true. A logarithmic function $\frac{1}{\ln(n)}$ grows faster $\frac{1}{n}$ regardless of constant placed on $\frac{1}{n}$.

Thus $\frac{1}{\ln(n)}$ not upper bound by $O(\frac{1}{n})$

My second question is

Show that $e^x-1=O(x^2)$ is not true.

But I think it is true because using a graph you see

using a constant c=3 $e^x-1=O(3x^2)$ for $x>0.413$ but I am not sure if this is correct.

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  • $\begingroup$ No, it is not correct. $e^x-1$ grows much faster, because it is "an exponential". . "Using a graph" is not really convincing here. $\endgroup$ – Dietrich Burde Sep 6 '15 at 18:38
  • $\begingroup$ Yes I think because e^x-1 is exponetial. $\endgroup$ – Fernando Martinez Sep 6 '15 at 18:41
  • $\begingroup$ What is the definition of big O that you are using? Normally, this is defined with the limit of a ratio, and this should be a useful way to proceed in both questions. $\endgroup$ – process91 Sep 6 '15 at 19:06
  • $\begingroup$ I have f(x)=0(g(x)) f(x)<=Cg(x) for all x>N $\endgroup$ – Fernando Martinez Sep 6 '15 at 19:41
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    $\begingroup$ Big O notation is the proper formalization for growing slower/faster than. So you should better use the formal definition, instead of intuitive arguments. In both cases, you could use l'hopital. $\endgroup$ – user251257 Sep 7 '15 at 1:17
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From the power series for $e^x$, $e^x > \frac{x^{n+1}}{(n+1)!} $, so $\frac{e^x}{x^n} >\frac{x}{(n+1)!} \to \infty $, so $e^x \ne O(x^n)$ for any $n$.

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