This is no ordinary Fibonacci spiral

Calculus Level 5

There is a race being held in the city of Mathville, the capital city of Mathematica. The race commences at the city centre. The competitors must first run 1 hectometre east, then 1 hectometre north, then 1 2 \frac{1}{2} hectometres west, then 1 3 \frac{1}{3} hectometres south, 1 5 \frac{1}{5} hectometres east and so forth where the n n th side of this spiral is 1 F n \frac{1}{F_n} hectometers, where F n F_n denotes the n n th Fibonacci number . Assuming the race goes on infinitely, the distance displaced by the competitors can be represented as l l metres . Find l \lfloor {l} \rfloor .


The answer is 99.

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1 solution

Let the starting point of the race the city center be the origin O ( 0 , 0 ) O(0,0) of the complex plane. Let east and west direction be the positive and negative real axis respectively, and north and south, the positive and negative imaginary axis respectively. Let each straight run or side length with direction be denoted by complex number a n a_n , where n n is a positive integer. Then a 1 = 1 a_1 = 1 , a 2 = 1 2 i a_2 = \frac 12 i , a 3 = 1 3 a_3 = - \frac 13 , a 4 = 1 5 i a_4 = - \frac 15 i , ... a n = i n 1 F n a_n = \frac {i^{n-1}}{F_n} .

Let s n = k = 1 n a k \displaystyle s_n = \sum_{k=1}^n a_k . We note that |s_n| is the distance between origin and the end of n n th side. Then, we have:

lim n s n = lim n k = 1 n i k 1 F k = k = 0 ( 1 ) k F 2 k + 1 + i k = 1 ( 1 ) k + 1 F 2 k By numerical method = 0.644358338 + 0.757204375 i (in hectometers) \begin{aligned} \lim_{n \to \infty} s_n & = \lim_{n \to \infty} \sum_{k=1}^n \frac {i^{k-1}}{F_k} \\ & = \sum_{k=0}^\infty \frac {(-1)^k}{F_{2k+1}} + i \sum_{k=1}^\infty \frac {(-1)^{k+1}}{F_{2k}} & \small \color{#3D99F6} \text{By numerical method} \\ & = 0.644358338 + 0.757204375i & \small \color{#3D99F6} \text{ (in hectometers)} \end{aligned}

Now l = 100 s = 100 × 0.994261602 l = 100 |s_\infty| = 100 \times 0.994261602 m, l = 99 \implies \lfloor l \rfloor = \boxed{99} m.

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