10.5. On the coordinate plane, a family of concentric circles centered at point is considered. a) Is there a circle in this family that has two rational points? b) Prove that there exists a circle in this family, inside which (i.e., inside the disk) there are exactly 2014 integer points. (A rational (integer) point is a point with rational (respectively, integer) coordinates.)
Problem 1037
Official solution
Answer: a) will not be found. Solution. a) Suppose, to the contrary, that there exist two rational points and on the circle of the given family. Then . Therefore, a rational number. If , then squaring the last equality, we get that is a rational number, which is false. If one of the differences, for example, , is 0, then with we obtain a contradiction with the irrationality of . Therefore, two different rational points on the circle cannot exist. b) Notice that inside a small-radius circle, there are no integer points (we can take a radius less than the distance from to the nearest integer point ). On the other hand, if we take a sufficiently large radius (for example, greater than 3000), then inside the circle there will be more than 2014 points. Since, by part a), when the radius is gradually increased, the jump in the number of integer points occurs only by one, there must necessarily come a moment when there will be exactly 2014 points inside the circle.