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Algebra Difficulty 5.7 AIME, harder Prove it Brazil

Let x1,x2,,x2004x_1, x_2, \dots, x_{2004} be a sequence of integer numbers such that xk+3=xk+2+xkxk+1x_{k+3} = x_{k+2} + x_k x_{k+1}, 1k20011 \le k \le 2001. Is it possible that more than half of the elements are negative?

Solution

The answer is yes. For instance, consider x0=1x_0 = -1, x1=nx_1 = -n and x2=n2x_2 = -n^2, nn sufficiently large. All xix_i's are polynomials in nn and, for all large nn its sign is equal to the sign of the coefficient of the term of greatest degree. Let a(xn)a(x_n) such term. Then
a(x0)=1,a(x1)=n,a(x2)=n2,a(x3)=n3,a(x4)=n3,a(x5)=n4,a(x6)=n5,a(x7)=n7 \begin{aligned} a(x_0) &= -1, & a(x_1) &= -n, & a(x_2) &= -n^2, & a(x_3) &= -n^3, \\ a(x_4) &= n^3, & a(x_5) &= n^4, & a(x_6) &= -n^5, & a(x_7) &= n^7 \end{aligned}
Let's prove by induction that, for k5k \ge 5, a(xk)=a(xk2)a(xk3)a(x_k) = a(x_{k-2})a(x_{k-3}), which is equivalent to prove that the degree of a(xk1)a(x_{k-1}) is less than the degree of a(xk2xk3)a(x_{k-2}x_{k-3}). This is true for k=5,6,7k = 5, 6, 7. Suppose that it is true for all kk less than mm. So the degree of a(xm1)=a(xm3)a(xm4)a(x_{m-1}) = a(x_{m-3})a(x_{m-4}) is less than the degree of a(xm2)a(xm3)a(x_{m-2})a(x_{m-3}) because by the induction hypothesis the degree of a(xl)a(x_l) is less than the degree of a(xl1)a(x_{l-1}) and we're done.
So the sign of xkx_k is equal to the sign of xk2xk3x_{k-2}x_{k-3} for k5k \ge 5. Thus the signs of the sequence xkx_k are
,,,,+,+,,+,,,,+,+,,+,+,,+,,,,+,+,,+,, -, -, -, -, +, +, -, +, -, -, -, +, +, -, +, +, -, +, -, -, -, +, +, -, +, -, \dots
which are composed of several cycles of the form ,,,+,+,,+-, -, -, +, +, -, +. Since 4 out of the 7 terms in each cycle are negative, the result follows.

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