Amazing \prod . <1>

Calculus Level 5

100 k = 1 3 1 + 2 cos ( π 3 k ) = ? \left \lfloor100\prod_{k=1}^{\infty}\frac{3}{1+2\cos\left(\frac{\pi}{3^{k}}\right)}\right \rfloor = ?

Notation: \lfloor\cdot\rfloor denotes the floor function .


The answer is 157.

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1 solution

Mark Hennings
Jun 3, 2018

Note that k = 1 K 3 1 + 2 cos π 3 k = 3 2 k = 2 K 3 e π i 3 k + 1 + e π i 3 k = 1 2 3 K k = 2 K e π i 3 k ( e π i 3 k 1 ) e π i 3 k 1 1 = 1 2 3 K k = 2 K sin π 2 × 3 k sin π 2 × 3 k 1 = 1 2 3 K sin π 2 × 3 K sin π 6 = 3 K sin π 2 × 3 K \begin{aligned} \prod_{k=1}^K \frac{3}{1 + 2\cos\frac{\pi}{3^k}} & = \; \tfrac32 \prod_{k=2}^K \frac{3}{e^{\frac{\pi i}{3^k}} + 1 + e^{\frac{-\pi i}{3^k}}} \; = \; \tfrac123^K\prod_{k=2}^K \frac{e^{\frac{\pi i}{3^k}}\big(e^{\frac{\pi i}{3^k}} - 1\big)}{e^{\frac{\pi i}{3^{k-1}}}-1} \\ & = \; \tfrac123^K \prod_{k=2}^K \frac{\sin \frac{\pi}{2\times3^k}}{\sin \frac{\pi}{2\times3^{k-1}}} \; = \; \tfrac123^K\frac{\sin\frac{\pi}{2\times3^K}}{\sin\frac{\pi}{6}} \; = \; 3^K \sin \tfrac{\pi}{2\times3^K} \end{aligned} From this it is clear that k = 1 3 1 + 2 cos π 3 k = lim K 3 K sin π 2 × 3 K = 1 2 π \prod_{k=1}^\infty \frac{3}{1 + 2\cos\frac{\pi}{3^k}} \; = \; \lim_{K \to \infty} 3^K \sin \tfrac{\pi}{2\times3^K} \; = \; \tfrac12\pi making the answer 50 π = 157 \lfloor 50\pi \rfloor = \boxed{157} .

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