Maths Olympiad Prep

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Algebra Difficulty 5.8 AIME, harder Prove it Ireland

Let (an)(a_n) be the "iterated Fibonacci sequence": an=f(f(n))a_n = f(f(n)), where f(1)=f(2)=1f(1) = f(2) = 1, and f(n+2)=f(n+1)+f(n)f(n+2) = f(n+1) + f(n). Prove that ana_n is a multiple of 144 whenever it is a multiple of 14.

Solution

This is actually true for the full Fibonacci sequence, not just the iterated one. Since the Fibonacci numbers depend only on the previous two values. Fibonacci modulo nn must be preperiodic with period at most n2n^2. In fact it is periodic for all of the values that we examine.

Calculation shows that f(n)f(n) mod 22 has a period of 33 and f(n)f(n) mod 77 has an "antiperiod" of 88 (meaning f(n+8)f(n)f(n+8) \equiv -f(n) mod 77), and so a period of 1616. Examination of the repeated values reveals that f(n)f(n) is a multiple of 22 exactly when nn is a multiple of 33, and a multiple of 77 exactly when nn is a multiple of 88. Thus 1414 divides f(n)f(n) exactly when 2424 divides nn.

Similarly, we see that both modulo 1616 and modulo 99, f(n)f(n) has a period of 2424 and is equivalent to 00 if and only if nn is a multiple of 1212. The result follows.

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