CombinatoricsDifficulty 5.4AIME, harderFind the answer
Let S be the set of 34 points in four-dimensional space where each coordinate is in {−1,0,1}. Let N be the number of sequences of points P1,P2,…,P2020 in S such that PiPi+1=2 for all 1≤i≤2020 and P1=(0,0,0,0). (Here P2021=P1.) Find the largest integer n such that 2n divides N.
A number or a short expression. Spacing and $ signs are ignored.
Solution
From (0,0,0,0) we have to go to (±1,±1,±1,±1), and from (1,1,1,1) (or any of the other similar points), we have to go to (0,0,0,0) or (−1,1,1,1) and its cyclic shifts. If ai is the number of ways to go from (1,1,1,1) to point of the form (±1,±1,±1,±1) in i steps, then we need to find ν2(16a2018). To find a recurrence relation for ai, note that to get to some point in (±1,±1,±1,±1), we must either come from a previous point of the form (±1,±1,±1,±1) or the point (0,0,0,0). In order to go to one point of the form (±1,±1,±1,±1) through (0,0,0,0) from the point (±1,±1,±1,±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) then we have 4 possible directions to go in. Therefore the recurrence relation for ai is ai=4ai−1+16ai−2. Solving the linear recurrence yields ai=51(2+25)i−51(2−25)i=4iFi+1 so it suffices to find ν2(F2019). We have Fn≡0,1,1,2,3,1(mod4) for n≡0,1,2,3,4,5(mod6), so ν2(F2019)=1, and the answer is 4+2⋅2018+1=4041.
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