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Algebra Difficulty 6.5 National olympiad Prove it Romania

Determine whether there exist a polynomial f(x1,x2)f(x_1, x_2) in two variables, with integer coefficients, and two points A=(a1,a2)A = (a_1, a_2) and B=(b1,b2)B = (b_1, b_2) in the plane, satisfying all the following conditions
(i) AA is an integer point (i.e., a1a_1 and a2a_2 are integers);
(ii) a1b1+a2b2=2010|a_1 - b_1| + |a_2 - b_2| = 2010;
(iii) f(n1,n2)>f(a1,a2)f(n_1, n_2) > f(a_1, a_2), for all integer points (n1,n2)(n_1, n_2) in the plane other than AA;
(iv) f(x1,x2)>f(b1,b2)f(x_1, x_2) > f(b_1, b_2), for all points (x1,x2)(x_1, x_2) in the plane other than BB.

Solutions — 2

Solution 1

The triple (f(x1,x2),A,B)(f(x_1, x_2), A, B) does exist, so the answer is yes.
Let A=O=(0,0)A = O = (0, 0), B=(x0,y0)=(2009+23,13)B = (x_0, y_0) = (2009 + \frac{2}{3}, \frac{1}{3}). The idea is to search for a polynomial ff such that f(x,y)=0f(x, y) = 0 is the equation of an ellipse centered at BB, passing through OO and with tangent line y=0y = 0 at OO. In fact, if ff is chosen like this, the ellipse f(x,y)=0f(x, y) = 0 is completely contained in the region {(x,y);0y23}\{(x, y) ; 0 \le y \le \frac{2}{3}\}, with OO the only integer point on the ellipse or in its interior; clearly, the absolute minimum of f(x,y)f(x, y) is attained at BB and f(x,y)f(x, y) is positive at all integer points other than OO. Therefore, we consider polynomials of the type
f(X,Y)=9M(Xx0)2+9N(Xx0)(Yy0)+9P(Yy0)2Q f(X, Y) = 9M(X - x_0)^2 + 9N(X - x_0)(Y - y_0) + 9P(Y - y_0)^2 - Q
where M,N,P,QM, N, P, Q are integers with M,P,Q,4MPN2>0M, P, Q, 4MP - N^2 > 0.
The condition that the ellipse f(x,y)=0f(x, y) = 0 passes through OO, with tangent line y=0y = 0 at OO, is expressed by
{60292M+6029N+PQ=026029M+N=0. \begin{cases} 6029^2 M + 6029N + P - Q = 0 \\ 2 \cdot 6029M + N = 0. \end{cases}

It is then sufficient to choose M=1M = 1, N=26029N = -2 \cdot 6029, PP any integer greater than 602926029^2 and Q=P60292Q = P - 6029^2.

Solution 2

(Alternative Solution. D. Schwarz)
Given any integer point A(a1,a2)A(a_1, a_2), there exist infinitely many points B(b1,b2)B(b_1, b_2) with b1,b2QZb_1, b_2 \in \mathbb{Q} \setminus \mathbb{Z}, and such that a1b1+a2b2=2010|a_1 - b_1| + |a_2 - b_2| = 2010, for example b1=a1+α+rb_1 = a_1 + \alpha + r, b2=a2+β+(1r)b_2 = a_2 + \beta + (1 - r), with α,βZ+\alpha, \beta \in \mathbb{Z}_+, rQ(0,1)r \in \mathbb{Q} \cap (0, 1), and α+β=2009\alpha + \beta = 2009. We now will consider polynomials of the type
f(X,Y)=N((Xa1)2+(Ya2)2+ε)((Xb1)2+(Yb2)2) f(X, Y) = N \left( (X - a_1)^2 + (Y - a_2)^2 + \varepsilon \right) \left( (X - b_1)^2 + (Y - b_2)^2 \right)
where εQ+\varepsilon \in \mathbb{Q}_+^* and NZ+N \in \mathbb{Z}_+^* large enough for f(X,Y)f(X, Y) to have integer coefficients.
One then has f(b1,b2)=0f(b_1, b_2) = 0, while f(x,y)>0f(x, y) > 0 for all points (x,y)(x, y) in the plane, other than BB.
One also then has f(a1,a2)=Nε((a1b1)2+(a2b2)2)f(a_1, a_2) = N\varepsilon((a_1 - b_1)^2 + (a_2 - b_2)^2), while one has, for all integer points (n1,n2)(n_1, n_2) in the plane, other than AA,
min{(n1a1)2+(n2a2)2+ε}=1+εandmin{(n1b1)2+(n2b2)2}=m, \min \{(n_1 - a_1)^2 + (n_2 - a_2)^2 + \varepsilon\} = 1 + \varepsilon \quad \text{and} \\ \min \{(n_1 - b_1)^2 + (n_2 - b_2)^2\} = m,
for some mQ(0,12]m \in \mathbb{Q} \cap (0, \frac{1}{2}] (for example m=12m = \frac{1}{2} when r=12r = \frac{1}{2}), therefore F(n1,n2)>N(1+ε)mF(n_1, n_2) > N(1+\varepsilon)m. In order to have f(n1,n2)>f(a1,a2)f(n_1, n_2) > f(a_1, a_2) it is thus enough that N(1+ε)mNε((a1b1)2+(a2b2)2)N(1+\varepsilon)m \ge N\varepsilon((a_1 - b_1)^2 + (a_2 - b_2)^2), therefore let us take
ε=m/((a1b1)2+(a2b2)2m)Q+, \varepsilon = m / ((a_1 - b_1)^2 + (a_2 - b_2)^2 - m) \in \mathbb{Q}_+^*,
and then choose some appropriate NN.

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