One more of those

Algebra Level 5

For w = e π i / 11 w=e^{\pi{i}/11} find

k = 0 11 ( w k 2 w k ) . \prod_{k=0}^{11}\left(w^k-2w^{-k}\right).


The answer is 2047.

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2 solutions

Otto Bretscher
May 12, 2015

Keep in mind throughout that w 22 = e 2 π i = 1. w^{22}=e^{2\pi{i}}=1.

Since the even powers of w w are the 11th roots of unity, we have z 11 1 = k = 0 10 ( z w 2 k ) z^{11}-1=\prod_{k=0}^{10}(z-w^{2k}) . We plug in z = 2 z=2 and find 2 11 1 = 2047 = k = 0 10 ( 2 w 2 k ) = k = 0 11 ( 2 w 2 k ) = k = 0 11 w k k = 0 11 ( 2 w k w k ) 2^{11}-1=2047=\prod_{k=0}^{10}(2-w^{2k})=\prod_{k=0}^{11}(2-w^{2k})=\prod_{k=0}^{11}w^k\prod_{k=0}^{11}(2w^{-k}-w^{k}) = w 11 × 12 2 k = 0 11 ( w k 2 w k ) = k = 0 11 ( w k 2 w k ) = 2047 =w^{\frac{11\times12}{2}}\prod_{k=0}^{11}(w^k-2w^{-k})=\prod_{k=0}^{11}(w^k-2w^{-k})=\boxed{2047}

You sure do love your roots of unites! Request: publish a book with problems accompanied by all their proofs. You have my endorsement!

Pi Han Goh - 6 years, 1 month ago

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I like to think that my Linear Algebra text ("Linear Algebra with Applications") has a lot of interesting problems, including many on roots of unity. I'm working on a calculus text now, and then I will take requests for the next one ;) It might be a travel book or a book on politics though... my other passions besides roots of unity.

Otto Bretscher - 6 years, 1 month ago

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May I know the author of the book you're referring to? The name is so generic.

Well, be sure to notify me when you publish it, I'll be your first customer!

Pi Han Goh - 6 years, 1 month ago

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@Pi Han Goh I must confess that I wrote the book myself ;)

Otto Bretscher - 6 years, 1 month ago
Aareyan Manzoor
Oct 10, 2015

Yes, that's another good way to do it (upvote)!

Otto Bretscher - 5 years, 8 months ago

I did same.

NICE PROBLEM!!

Dev Sharma - 5 years, 5 months ago

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