Solution:
The elements of a number bracelet N will be denoted N0,N1,N2,…; N0 and N1 being the starting numbers. Let F be the bracelet with starting numbers F0=0,F1=1. All congruences are modulo m unless otherwise noted.
Since there are only m2 possible pairs (Nk,Nk+1), there must be repetition, so that
(Nk,Nk+1)=(Np+k,Np+k+1)
for some p≥1 and k. If this is true for one k, it must be true for the next k, since Nk+1=Np+k+1 by hypothesis and Nk+2≡Nk+Nk+1=Np+k+Np+k+1≡Np+k+2, and so it is true for all k bigger than that k. It must also be true for the previous k, since Nk=Np+k by hypothesis and Nk−1≡Nk+1−Nk=Np+k+1−Np+k≡Np+k−1, and so it is true for all k smaller than that k. Therefore it is true for all k.
Let PN, the period of N, be the smallest p such that this is true for some k (and therefore for all k).
It is clear that if 0≤i,j<P(N),i=j, then (Ni,Ni+1)=(Nj,Nj+1). Therefore, among the terms N0 through NPN, there is no repetition even of pairs of adjacent terms. But because of the property of PN, (N0,N1)=(NPN,NPN+1) and the repetition will continue. Thus the number bracelet N consists of a single loop of PN elements.
We will prove by induction that for all n≥1,
Nn≡N0Fn−1+N1Fn.
The case n=1 is trivial, and n=2 follows directly from the definition of N. If this is true for n=k and n=k−1, Nk+1≡Nk+Nk−1≡N0Fk−1+N1Fk+N0Fk−2+N1Fk−1=N0Fk+N1Fk+1 so it is also true for n=k+1.
It is clear from the circularity of the number bracelet that Nk+R=Nk for all k≥1 if and only if PN∣R. Since Nk+PF=N0FPF+k−1+N1FPF+k=N0Fk−1+N1Fk=Nk for all k≥1, PN∣PF. This completes the proof.