Maths Olympiad Prep

Library / /29 of 64

Algebra Difficulty 7.8 National olympiad, round 2 Find the answer

Let FmF_m be the mmth Fibonacci number, defined by F1=F2=1F_1 = F_2 = 1 and Fm=Fm1+Fm2F_m = F_{m-1} + F_{m-2} for all m3m \geq 3. Let p(x)p(x) be the polynomial of degree 10081008 such that p(2n+1)=F2n+1p(2n+1) = F_{2n+1} for n=0,1,2,,1008n=0,1,2,\dots,1008. Find integers jj and kk such that p(2019)=FjFkp(2019) = F_j - F_k.

A number or a short expression. Spacing and $ signs are ignored.

Solution

We prove that (j,k)=(2019,1010)(j,k) = (2019, 1010) is a valid solution. More generally, let p(x)p(x) be the polynomial of degree NN such that p(2n+1)=F2n+1p(2n+1) = F_{2n+1} for 0nN0 \leq n \leq N. We will show that p(2N+3)=F2N+3FN+2p(2N+3) = F_{2N+3}-F_{N+2}.

Define a sequence of polynomials p0(x),,pN(x)p_0(x),\ldots,p_N(x) by p0(x)=p(x)p_0(x) = p(x) and pk(x)=pk1(x)pk1(x+2)p_k(x) = p_{k-1}(x)-p_{k-1}(x+2) for k1k \geq 1. Then by induction on kk, it is the case that pk(2n+1)=F2n+1+kp_k(2n+1) = F_{2n+1+k} for 0nNk0 \leq n \leq N-k, and also that pkp_k has degree (at most) NkN-k for k1k \geq 1. Thus pN(x)=FN+1p_N(x) = F_{N+1} since pN(1)=FN+1p_N(1) = F_{N+1} and pNp_N is constant.

We now claim that for 0kN0\leq k\leq N, pNk(2k+3)=j=0kFN+1+jp_{N-k}(2k+3) = \sum_{j=0}^k F_{N+1+j}. We prove this again by induction on kk: 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 p(2N+3)=p0(2N+3)=j=0NFN+1+jp(2N+3) = p_0(2N+3) = \sum_{j=0}^N F_{N+1+j}.

Now one final induction shows that j=1mFj=Fm+21\sum_{j=1}^m F_j = F_{m+2}-1, and so p(2N+3)=F2N+3FN+2p(2N+3) = F_{2N+3}-F_{N+2}, as claimed. In the case N=1008N=1008, we thus have p(2019)=F2019F1010p(2019) = F_{2019} - F_{1010}.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.