Let be the th Fibonacci number, defined by and for all . Let be the polynomial of degree such that for . Find integers and such that .
Solution
We prove that is a valid solution. More generally, let be the polynomial of degree such that for . We will show that .
Define a sequence of polynomials by and for . Then by induction on , it is the case that for , and also that has degree (at most) for . Thus since and is constant.
We now claim that for , . We prove this again by induction on : for the induction step, we have \begin{align*} p_{N-k}(2k+3) &= p_{N-k}(2k+1)+p_{N-k+1}(2k+1) \\ &= F_{N+1+k}+\sum_{j=0}^{k-1} F_{N+1+j}. \end{align*} Thus we have .
Now one final induction shows that , and so , as claimed. In the case , we thus have .
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