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Old 2020-10-14, 17:49   #11
Nick's Avatar
Dec 2012
The Netherlands

1,621 Posts

Originally Posted by bur View Post
Having looked at the proof, what I don't get is, why is 2^{(2r+1)s}=(2^s + 1)(...)?
It's an example of what's called a telescopic sum: if you multiply out the brackets on the right hand side, then all terms cancel except for the first and the last.
If it's not clear, try writing it out with a small example such as r=3.
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