Maths Olympiad Prep

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Algebra Difficulty 5.8 AIME, harder Prove it United States

Problem:

A number bracelet in base mm is made by choosing two nonnegative integers less than mm (not both 00) and continuing in a clockwise loop, each succeeding number being the mod mm sum of its two predecessors. The figure is closed up as soon as it starts to repeat. The figure to the right shows two number bracelets in base 1010, starting with the pairs (1,3)(1,3), and (2,2)(2,2), respectively. Prove that the lengths of all number bracelets in a given base are divisors of the length of the number bracelet beginning with (0,1)(0,1).

Solution

Solution:

The elements of a number bracelet NN will be denoted N0,N1,N2,N_{0}, N_{1}, N_{2}, \ldots; N0N_{0} and N1N_{1} being the starting numbers. Let FF be the bracelet with starting numbers F0=0,F1=1F_{0}=0, F_{1}=1. All congruences are modulo mm unless otherwise noted.

Since there are only m2m^{2} possible pairs (Nk,Nk+1)(N_{k}, N_{k+1}), there must be repetition, so that
(Nk,Nk+1)=(Np+k,Np+k+1) (N_{k}, N_{k+1}) = (N_{p+k}, N_{p+k+1})
for some p1p \geq 1 and kk. If this is true for one kk, it must be true for the next kk, since Nk+1=Np+k+1N_{k+1} = N_{p+k+1} by hypothesis and Nk+2Nk+Nk+1=Np+k+Np+k+1Np+k+2N_{k+2} \equiv N_{k} + N_{k+1} = N_{p+k} + N_{p+k+1} \equiv N_{p+k+2}, and so it is true for all kk bigger than that kk. It must also be true for the previous kk, since Nk=Np+kN_{k} = N_{p+k} by hypothesis and Nk1Nk+1Nk=Np+k+1Np+kNp+k1N_{k-1} \equiv N_{k+1} - N_{k} = N_{p+k+1} - N_{p+k} \equiv N_{p+k-1}, and so it is true for all kk smaller than that kk. Therefore it is true for all kk.

Let PNP_{N}, the period of NN, be the smallest pp such that this is true for some kk (and therefore for all kk).

It is clear that if 0i,j<P(N),ij0 \leq i, j < P(N), i \neq j, then (Ni,Ni+1)(Nj,Nj+1)(N_{i}, N_{i+1}) \neq (N_{j}, N_{j+1}). Therefore, among the terms N0N_{0} through NPNN_{P_{N}}, there is no repetition even of pairs of adjacent terms. But because of the property of PNP_{N}, (N0,N1)=(NPN,NPN+1)(N_{0}, N_{1}) = (N_{P_{N}}, N_{P_{N}+1}) and the repetition will continue. Thus the number bracelet NN consists of a single loop of PNP_{N} elements.

We will prove by induction that for all n1n \geq 1,
NnN0Fn1+N1Fn. N_{n} \equiv N_{0} F_{n-1} + N_{1} F_{n}.
The case n=1n=1 is trivial, and n=2n=2 follows directly from the definition of NN. If this is true for n=kn=k and n=k1n=k-1, Nk+1Nk+Nk1N0Fk1+N1Fk+N0Fk2+N1Fk1=N0Fk+N1Fk+1N_{k+1} \equiv N_{k} + N_{k-1} \equiv N_{0} F_{k-1} + N_{1} F_{k} + N_{0} F_{k-2} + N_{1} F_{k-1} = N_{0} F_{k} + N_{1} F_{k+1} so it is also true for n=k+1n=k+1.

It is clear from the circularity of the number bracelet that Nk+R=NkN_{k+R} = N_{k} for all k1k \geq 1 if and only if PNRP_{N} \mid R. Since Nk+PF=N0FPF+k1+N1FPF+k=N0Fk1+N1Fk=NkN_{k+P_{F}} = N_{0} F_{P_{F}+k-1} + N_{1} F_{P_{F}+k} = N_{0} F_{k-1} + N_{1} F_{k} = N_{k} for all k1k \geq 1, PNPFP_{N} \mid P_{F}. This completes the proof.

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