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This video is my best shot at animating and explaining my favourite proof that pi is irrational. It is due to the Swiss mathematician Johann Lambert who published it over 250 years ago.

The original writeup by Lambert is 58 pages long and definitely not for the faint of heart (http://www.kuttaka.org/~JHL/L1768b.pdf). On the other hand, among all the proofs of the irrationality of pi, Lambert's proof is probably the most "natural" one, the one that's easiest to motivate and explain, and one that's ideally suited for the sort of animations that I do.

Anyway it's been an absolute killer to put this video together and overall this is probably the most ambitious topic I've tackled so far. I really hope that a lot of you will get something out of it. If you do please let me know :) Also, as usual, please consider contributing subtitles in your native language (English and Russian are under control, but everything else goes).

One of the best short versions of Lambert's proof is contained in the book Autour du nombre pi by JeanPierre Lafon and Pierre Eymard. In particular, in it the authors calculate an explicit formula for the nth partial fraction of Lambert's tan x formula; here is a scan with some highlighting by me: http://www.qedcat.com/misc/chopped.png
Have a close look and you'll see that as n goes to infinity all the highlighted terms approach 1. What's left are the Maclaurin series for sin x on top and that for cos x at the bottom and this then goes a long way towards showing that those partial fractions really tend to tan x.

There is a good summary of other proofs for the irrationality of pi on this wiki page: https://en.wikipedia.org/wiki/Proof_t...

Today's main tshirt I got from from Zazzle:
https://www.zazzle.com.au/25_dec_31_o...
(there are lots of places that sell "HO cubed" tshirts)

lf you liked this video maybe also consider checking out some of my other videos on irrational and transcendental numbers and on continued fractions and other infinite expressions. The video on continued fractions that I refer to in this video is my video on the most irrational number:    • Infinite fractions and the most irrat...  

Special thanks to my friend Marty Ross for lots of feedback on the slideshow and some goodhumoured heckling while we were recording the video. Thank you also to Danil Dimitriev for his ongoing Russian support of this channel.

Merry Christmas!