Now, what familiar equations can I use?

Calculus Level 3

n = 1 ( 1 1 e ) n n = ? \large\displaystyle\sum_{n=1}^{\infty}\frac{\left(1-\frac{1}{e}\right)^n}{n}=\ ?

With e = m = 0 1 m ! 2.71828 \displaystyle e = \sum_{m=0}^\infty \frac1{m!} \approx 2.71828 .


The answer is 1.

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1 solution

The Maclaurin series for ln ( 1 x ) -\ln(1 - x) is n = 1 x n n \displaystyle\sum_{n=1}^{\infty} \dfrac{x^{n}}{n} for x < 1. |x| \lt 1.

In this case we have x = 1 1 e , x = 1 - \dfrac{1}{e}, and thus the given series has the value

ln ( 1 ( 1 1 e ) ) = ln ( 1 e ) = ln ( e 1 ) = 1 . -\ln\left(1 - \left(1 - \dfrac{1}{e}\right)\right) = -\ln\left(\dfrac{1}{e}\right) = -\ln(e^{-1}) = \boxed{1}.

Proof of series expansion: Note first that the derivative of ln ( 1 x ) -\ln(1 - x) is 1 1 x . \dfrac{1}{1 - x}.

So for x < 1 |x| \lt 1 we have that ln ( 1 x ) = 1 1 x d x = -\ln(1 - x) = \displaystyle\int \dfrac{1}{1 - x} dx =

( n = 0 x n ) d x = n = 0 ( x n d x ) = n = 0 x n + 1 n + 1 + C = n = 1 x n n + 1 + C . \displaystyle\int \left(\sum_{n=0}^{\infty} x^{n} \right) dx = \displaystyle\sum_{n=0}^{\infty} \left(\int x^{n} dx \right) = \sum_{n=0}^{\infty} \dfrac{x^{n+1}}{n + 1} + C = \sum_{n=1}^{\infty} \dfrac{x^{n}}{n + 1} + C.

Now since ln ( 1 ) = 0 \ln(1) = 0 we see that C = 0 , C = 0, resulting in the aforementioned series representation.

Comments: For a discussion on the permissibility of exchanging summation and integral signs, see this link .

In general, for y 1 y \ge 1 we have that n = 1 ( 1 1 y ) n n = ln ( y ) . \displaystyle\sum_{n=1}^{\infty} \dfrac{(1 - \frac{1}{y})^{n}}{n} = \ln(y).

Moderator note:

Nicely done! Thanks for adding the generalization.

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