Determine whether there exist a polynomial in two variables, with integer coefficients, and two points and in the plane, satisfying all the following conditions
(i) is an integer point (i.e., and are integers);
(ii) ;
(iii) , for all integer points in the plane other than ;
(iv) , for all points in the plane other than .
Solutions — 2
Solution 1
The triple does exist, so the answer is yes.
Let , . The idea is to search for a polynomial such that is the equation of an ellipse centered at , passing through and with tangent line at . In fact, if is chosen like this, the ellipse is completely contained in the region , with the only integer point on the ellipse or in its interior; clearly, the absolute minimum of is attained at and is positive at all integer points other than . Therefore, we consider polynomials of the type
where are integers with .
The condition that the ellipse passes through , with tangent line at , is expressed by
It is then sufficient to choose , , any integer greater than and .
Solution 2
(Alternative Solution. D. Schwarz)
Given any integer point , there exist infinitely many points with , and such that , for example , , with , , and . We now will consider polynomials of the type
where and large enough for to have integer coefficients.
One then has , while for all points in the plane, other than .
One also then has , while one has, for all integer points in the plane, other than ,
for some (for example when ), therefore . In order to have it is thus enough that , therefore let us take
and then choose some appropriate .