Number theoryDifficulty 5.1AIME, harderFind the answer
The Fibonacci sequence is defined as follows: F0=0,F1=1, and Fn=Fn−1+Fn−2 for all integers n≥2. Find the smallest positive integer m such that Fm≡0(mod127) and Fm+1≡1(mod127).
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Solution
First, note that 5 is not a quadratic residue modulo 127. We are looking for the period of the Fibonacci numbers mod127. Let p=127. We work in Fp2 for the remainder of this proof. Let α and β be the roots of x2−x−1. Then we know that Fn=α−βαn−βn. Note that since x→xp is an automorphism and since automorphisms cycle the roots of a polynomial we have that αp=β and βp=α. Then Fp=α−βαp−βp=−1 and Fp+1=α−βαβ−βα=0 and similarly we obtain F2p+1=1 and F2p+2=0. Thus since 2p+2 is a power of 2 and since the period does not divide p+1, we must have the answer is 2p+2=256.
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