It's time to solve combinatorics 2

S n = k = 0 n ( n k ) ω k . S_n=\large \displaystyle \sum_{ k=0}^{n}\dbinom{n}{k}{\omega}^k. If n n is a positive integer and ω 1 \omega \neq 1 is a cube root of unity, find the number of possible values of the expression above.


The answer is 6.

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2 solutions

Otto Bretscher
May 13, 2016

Note that ω + 1 = e ± i π / 3 \omega+1=e^{\pm i\pi/3} , a primitive sixth root of unity. Thus S n = ( ω + 1 ) n S_n=(\omega+1)^n can attain 6 \boxed{6} values, namely, all sixth roots of unity.

Ankit Nigam
May 16, 2016

S n = k = 0 n ( n k ) ω k = ( 1 + ω ) n = ( ω 2 ) n ( 1 + ω + ω 2 = 0 ) S_n=\displaystyle \sum_{ k=0}^{n}\dbinom{n}{k}{\omega}^k = (1 + \omega)^n = (- \omega^2)^n\ (\because 1 + \omega + \omega^2 = 0)

check the cases ans will be 6 \boxed{6} .

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