We call two points in the Cartesian plane rivals if the difference between their and coordinates are both integers. We call two circles in the Cartesian plane enemies if their centers are distinct and rivals of each other.
Let be a set of circles in the Cartesian plane such that any two circles in are enemies of each other. Also, suppose all circles in have exactly lattice points in their interior. What is the maximum number of elements can have?
Details and assumptions
The circles must have distinct centers.
A lattice point is a point whose and coordinates are both integers.
The points lying on the circumference of the circles are counted in the total count of lattice points in the interior. Also, if the center of the circle is a lattice point, it is counted in the total count of lattice points in the interior.
As an explicit example, the points are rivals of each other, because
The circle in the following figure has lattice points in its interior (the points marked blue).
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Let D ( P , Q ) denote the distance between points P and Q in the Cartesian plane.
Consider a point P ( α , β ) whose coordinates are distinct and both irrational.
Claim: The distance from P to all lattice points are distinct.
Proof: Let M ( p , q ) and N ( r , s ) be two lattice points. We have to show that D ( P , M ) = D ( P , N ) ⟹ M = N . Indeed, note that D ( P , M ) = D ( P , N ) ⟹ ( α − p ) 2 + ( β − q ) 2 = ( α − r ) 2 + ( β − s ) 2 ⟹ α ( 2 p − 2 r ) + β ( 2 q − 2 s ) = p 2 + q 2 − r 2 − s 2 . Since α , β are distinct irrational numbers, for the LHS to be an integer, we must have 2 p − 2 r = 0 , 2 q − 2 s = 0 ⟹ p = q , q = s ⟹ M = N , as desired. ■
Now, let T n denote the n th closest point to P . Consider any circle centered at P and radius strictly between D ( P , T 2 0 1 4 ) and D ( P , T 2 0 1 5 ) . This circle contains precisely 2 0 1 4 lattice points in its interior. So, for all points with distinct irrational coordinates, we can find a circle with exactly 2 0 1 4 lattice points in its interior. There exist infinitely many points with distinct irrational coordinates which are pairwise rivals (for an explicit construction, consider { ( 2 + i , 3 + i ) } i = 1 ∞ ). Therefore, our answer is ∞ .