Maths Olympiad Prep

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Combinatorics Difficulty 5.4 AIME, harder Find the answer

Let SS be the set of 343^{4} points in four-dimensional space where each coordinate is in {1,0,1}\{-1,0,1\}. Let NN be the number of sequences of points P1,P2,,P2020P_{1}, P_{2}, \ldots, P_{2020} in SS such that PiPi+1=2P_{i} P_{i+1}=2 for all 1i20201 \leq i \leq 2020 and P1=(0,0,0,0)P_{1}=(0,0,0,0). (Here P2021=P1P_{2021}=P_{1}.) Find the largest integer nn such that 2n2^{n} divides NN.

A number or a short expression. Spacing and $ signs are ignored.

Solution

From (0,0,0,0)(0,0,0,0) we have to go to (±1,±1,±1,±1)( \pm 1, \pm 1, \pm 1, \pm 1), and from (1,1,1,1)(1,1,1,1) (or any of the other similar points), we have to go to (0,0,0,0)(0,0,0,0) or (1,1,1,1)(-1,1,1,1) and its cyclic shifts. If aia_{i} is the number of ways to go from (1,1,1,1)(1,1,1,1) to point of the form (±1,±1,±1,±1)( \pm 1, \pm 1, \pm 1, \pm 1) in ii steps, then we need to find ν2(16a2018)\nu_{2}\left(16 a_{2018}\right). To find a recurrence relation for aia_{i}, note that to get to some point in (±1,±1,±1,±1)( \pm 1, \pm 1, \pm 1, \pm 1), we must either come from a previous point of the form (±1,±1,±1,±1)( \pm 1, \pm 1, \pm 1, \pm 1) or the point (0,0,0,0)(0,0,0,0). In order to go to one point of the form (±1,±1,±1,±1)( \pm 1, \pm 1, \pm 1, \pm 1) through (0,0,0,0)(0,0,0,0) from the point (±1,±1,±1,±1)( \pm 1, \pm 1, \pm 1, \pm 1), we have one way of going to the origin and 16 ways to pick which point we go to after the origin. Additionally, if the previous point we visit is another point of the form (±1,±1,±1,±1)( \pm 1, \pm 1, \pm 1, \pm 1) then we have 4 possible directions to go in. Therefore the recurrence relation for aia_{i} is ai=4ai1+16ai2a_{i}=4 a_{i-1}+16 a_{i-2}. Solving the linear recurrence yields ai=15(2+25)i15(225)i=4iFi+1a_{i}=\frac{1}{\sqrt{5}}(2+2 \sqrt{5})^{i}-\frac{1}{\sqrt{5}}(2-2 \sqrt{5})^{i}=4^{i} F_{i+1} so it suffices to find ν2(F2019)\nu_{2}\left(F_{2019}\right). We have Fn0,1,1,2,3,1(mod4)F_{n} \equiv 0,1,1,2,3,1(\bmod 4) for n0,1,2,3,4,5(mod6)n \equiv 0,1,2,3,4,5(\bmod 6), so ν2(F2019)=1\nu_{2}\left(F_{2019}\right)=1, and the answer is 4+22018+1=40414+2 \cdot 2018+1=4041.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.