Precise Control Over The Number Of Lattice Points

Geometry Level 4

We call two points in the Cartesian plane rivals if the difference between their x x and y y coordinates are both integers. We call two circles in the Cartesian plane enemies if their centers are distinct and rivals of each other.

Let S S be a set of circles in the Cartesian plane such that any two circles in S S are enemies of each other. Also, suppose all circles in S S have exactly 2014 2014 lattice points in their interior. What is the maximum number of elements S S can have?

Details and assumptions

  • The circles must have distinct centers.

  • A lattice point is a point whose x x and y y 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 ( 3.1 , 4.3 ) , ( 5.1 , 6.3 ) (3.1, 4.3), (5.1, 6.3) are rivals of each other, because 5.1 3.1 = 2 Z , 6.3 4.3 = 2 Z . 5.1-3.1= 2 \in \mathbb{Z}, 6.3 - 4.3 = 2 \in \mathbb{Z}.

  • The circle in the following figure has 37 37 lattice points in its interior (the points marked blue).

0 1 201 4 2 + 2014 1 2014^2+2014-1 \infty

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1 solution

Let D ( P , Q ) D(P,Q) denote the distance between points P P and Q Q in the Cartesian plane.


Consider a point P ( α , β ) P (\alpha , \beta ) whose coordinates are distinct and both irrational.

Claim: The distance from P P to all lattice points are distinct.

Proof: Let M ( p , q ) M (p, q) and N ( r , s ) N( r,s) be two lattice points. We have to show that D ( P , M ) = D ( P , N ) M = N . D (P, M) = D(P,N) \implies 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 . D(P,M) = D(P,N) \implies (\alpha - p)^2 + (\beta - q)^2 = (\alpha - r)^2 + (\beta - s)^2 \\ \implies \alpha (2p - 2r) + \beta (2q - 2s) = p^2+q^2-r^2-s^2. Since α , β \alpha, \beta 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 , 2p - 2r= 0, 2q- 2s= 0 \implies p=q, q=s \implies M=N, as desired. \blacksquare


Now, let T n T_n denote the n th n^{\text{th}} closest point to P . P. Consider any circle centered at P P and radius strictly between D ( P , T 2014 ) D(P,T_{2014}) and D ( P , T 2015 ) . D(P,T_{2015}). This circle contains precisely 2014 2014 lattice points in its interior. So, for all points with distinct irrational coordinates, we can find a circle with exactly 2014 2014 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 \{ (\sqrt{2} + i, \sqrt{3} + i) \}_{i=1}^{\infty} ). Therefore, our answer is . \boxed{\infty}.

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