An alternate power series representation for ln(x)
#5
(05/07/2011, 11:20 PM)JmsNxn Wrote: \( \ln(x) = a + \sum_{n=1}^{\infty} \frac{(-1)^{n+1}}{ne^{an}}(x-e^a)^n \)

But this follows directly from the definition of the logarithm, by the following equivalent transformations:
\( \begin{eqnarray}
\ln(x) &=& a + \sum_{n=1}^{\infty} \frac{(-1)^{n+1}}{ne^{an}}(x-e^a)^n\\
&=& a + \sum_{n=1}^{\infty} \frac{(-1)^{n+1}}{n}(x/e^a-1)^n\\
\ln(e^a y) &=& a + \sum_{n=1}^{\infty} \frac{(-1)^{n+1}}{n}(y-1)^n\\
a + \ln(y) &=& a + \sum_{n=1}^{\infty} \frac{(-1)^{n+1}}{n}(y-1)^n\\
\ln(y) &=& \sum_{n=1}^{\infty} \frac{(-1)^{n+1}}{n}(y-1)^n
\end{eqnarray} \)

Quote:sadly, doesn't converge for values less than 1
The \( \ln \) series converges for \( |y-1|<1 \), hence your series converges for \( |x/e^a-1|<1 \), this should include all values \( x\in (0,2e^a) \)?
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Messages In This Thread
RE: An alternate power series representation for ln(x) - by bo198214 - 05/08/2011, 01:38 PM

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