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Number theory Difficulty 6.3 National Olympiad Prove it Ukraine

Grasshopper is sitting at point OO of coordinate line. He makes 20162016 jumps in positive direction. Firstly his jump has length 11, every next jump is kNk \in \mathbb{N} times longer than the previous one. There are holes in every point with coordinate 2016l2016l, lNl \in \mathbb{N}. Determine kk for which the grasshopper will make all the jumps and will not jump into hole.

Solution

a1=1,an=1+k+k2++kn1,n=2,,2016. a_1 = 1, \quad a_n = 1 + k + k^2 + \dots + k^{n-1}, \quad n = 2, \ldots, 2016.
We have to find such kk, for which ana_n is not divisible by 20162016.

Let us suggest that (k,2016)=d>1(k, 2016) = d > 1. As we see, every coordinate after the jump has residue 11 modulo dd thus cannot be divisible by 20162016. So all such kk are the answers.

If (k,2016)=1(k, 2016) = 1, let us look at all the residues of ana_n modulo 20162016. If at least one value has residue 00 then kk does not satisfy the condition. Let us suggest that there are no zeroes, then at least two residues are the same. Let them be ama_m and ala_l, m>lm > l. Their difference amala_m - a_l is divisible by 20162016, but then
amal=(1+k+k2++km1)(1+k+k2++kl1)= a_m - a_l = (1 + k + k^2 + \dots + k^{m-1}) - (1 + k + k^2 + \dots + k^{l-1}) =
kl(1+k+k2++kml1)=klaml. k^l (1 + k + k^2 + \dots + k^{m-l-1}) = k^l a_{m-l}.
Thus amla_{m-l} is divisible by 20162016, and we have a contradiction.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.