Log 2 equals to 0

Calculus Level 2

Consider the harmonic series: x = 1 1 x = 1 1 + 1 2 + 1 3 + 1 4 + 1 5 + \displaystyle \sum_{x=1}^\infty \frac1x = \frac11 + \frac12 + \frac13 + \frac14 + \frac15 + \ldots

I'm going to rearrange the terms to obtain ln ( 2 ) = 0 \ln(2) = 0 by first grouping the terms in pairs:

x = 1 1 x = ( 1 1 + 1 2 ) + ( 1 3 + 1 4 ) + ( 1 5 + 1 6 ) + = x = 1 ( 1 2 x 1 + 1 2 x ) = x = 1 ( 2 x + 2 x 1 2 x ( 2 x 1 ) ) = x = 1 ( 2 ( 2 x 1 ) + 1 2 x ( 2 x 1 ) ) = x = 1 [ 2 ( 2 x 1 ) 2 x ( 2 x 1 ) + 1 2 x ( 2 x 1 ) ] = x = 1 [ 1 x + 1 2 x ( 2 x 1 ) ] x = 1 1 x = x = 1 1 x + x = 1 1 2 x ( 2 x 1 ) 0 = x = 1 1 2 x ( 2 x 1 ) 0 = x = 1 ( 1 2 x 1 2 x 1 ) 0 = 1 1 2 + 1 3 1 4 + 1 5 = ln ( 2 ) \begin{aligned} \displaystyle \sum_{x=1}^\infty \frac1x &=& \left( \frac11 + \frac12 \right)+\left( \frac13 + \frac14 \right)+\left( \frac15 + \frac16 \right)+\ldots \\ \displaystyle &=& \sum_{x=1}^\infty \left( \frac1{2x-1} + \frac1{2x} \right) \\ \displaystyle &=& \sum_{x=1}^\infty \left( \frac{2x+2x-1}{2x(2x-1)} \right) \\ \displaystyle &=& \sum_{x=1}^\infty \left( \frac{2(2x-1) + 1}{2x(2x-1)} \right) \\ \displaystyle &=& \sum_{x=1}^\infty \left [ \frac{2(2x-1)}{2x(2x-1)} + \frac1{2x(2x-1)} \right] \\ \displaystyle &=& \sum_{x=1}^\infty \left [ \frac1x + \frac1{2x(2x-1)} \right] \\ \displaystyle \xcancel{\sum_{x=1}^\infty \frac1x} &=& \xcancel{\sum_{x=1}^\infty \frac1x} + \sum_{x=1}^\infty \frac1{2x(2x-1)} \\ \displaystyle 0 &=& \sum_{x=1}^\infty \frac1{2x(2x-1)} \\ \displaystyle 0 &=& \sum_{x=1}^\infty \left ( \frac1{2x} - \frac1{2x-1} \right ) \\ \displaystyle 0 &=& 1 - \frac12 + \frac13 - \frac14 + \frac15 - \ldots = \ln(2) \\ \end{aligned}

Above shows 10 steps for the supposed claim of ln ( 2 ) = 0 \ln(2) = 0 . How many of these steps are incorrect?

Adapted from Simon Singh's blog .
Scene is at 6 minute 46 seconds of the episode Sky Police from The Simpsons.
No copyright infringement intended.
2 3 4 5

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1 solution

Jake Lai
Jul 4, 2015

Apparently the Simpsons did a flawed proof of how ln 2 = 0 \ln 2 = 0 independently of my own flawed proof. Oh well.

As Simon Singh's blog says, you cannot simply cancel infinities on both sides. This is perhaps related to the Riemann series theorem .

Also, the ninth step's partial fraction decomposition is very silly. And wrong.

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