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Algebra Difficulty 5.7 AIME, harder Find the answer

Compute k=11007(cos(πk1007))2014\sum_{k=1}^{1007}\left(\cos \left(\frac{\pi k}{1007}\right)\right)^{2014}.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Our desired expression is 122014k=11007(ωk+ωk)2014\frac{1}{2^{2014}} \sum_{k=1}^{1007}\left(\omega^{k}+\omega^{-k}\right)^{2014}. Using binomial expansion and switching the order of the resulting summation, this is equal to 122014j=02014(2014j)k=11007(ω20142j)k\frac{1}{2^{2014}} \sum_{j=0}^{2014}\binom{2014}{j} \sum_{k=1}^{1007}\left(\omega^{2014-2j}\right)^{k}. Note that unless ω20142j=1\omega^{2014-2j}=1, the summand k=11007(ω20142j)k\sum_{k=1}^{1007}\left(\omega^{2014-2j}\right)^{k} is the sum of roots of unity spaced evenly around the unit circle in the complex plane (in particular the 1007th, 19th, and 53rd roots of unity), so it is zero. Thus, we must only sum over those jj for which ω20142j=1\omega^{2014-2j}=1, which holds for j=0,1007,2014j=0,1007,2014. This yields the answer 122014(1007+1007(20141007)+1007)=2014(1+(20131007))22014\frac{1}{2^{2014}}\left(1007+1007\binom{2014}{1007}+1007\right)=\frac{2014\left(1+\binom{2013}{1007}\right)}{2^{2014}}.

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