For w = e π i / 1 1 find
k = 0 ∏ 1 1 ( w k − 2 w − k ) .
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You sure do love your roots of unites! Request: publish a book with problems accompanied by all their proofs. You have my endorsement!
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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.
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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!
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@Pi Han Goh – I must confess that I wrote the book myself ;)
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Keep in mind throughout that w 2 2 = e 2 π i = 1 .
Since the even powers of w are the 11th roots of unity, we have z 1 1 − 1 = ∏ k = 0 1 0 ( z − w 2 k ) . We plug in z = 2 and find 2 1 1 − 1 = 2 0 4 7 = k = 0 ∏ 1 0 ( 2 − w 2 k ) = k = 0 ∏ 1 1 ( 2 − w 2 k ) = k = 0 ∏ 1 1 w k k = 0 ∏ 1 1 ( 2 w − k − w k ) = w 2 1 1 × 1 2 k = 0 ∏ 1 1 ( w k − 2 w − k ) = k = 0 ∏ 1 1 ( w k − 2 w − k ) = 2 0 4 7