HMMT 二月 2010 · GEN1 赛 · 第 8 题

英文解析

8. [ 6 ] A sphere is the set of points at a fixed positive distance r from its center. Let S be a set of 2010 dimensional spheres. Suppose that the number of points lying on every element of S is a finite numbern . Find the maximum possible value of n .
   Answer: 2 The answer is 2 for any number of dimensions. We prove this by induction on thedimension.
   Note that 1-dimensional spheres are pairs of points, and 2-dimensional spheres are circles.
   Base case, d = 2: The intersection of two circles is either a circle (if the original circles are identical, andin the same place), a pair of points, a single point (if the circles are tangent), or the empty set. Thus,
   in dimension 2, the largest finite number of intersection points is 2, because the number of pairwiseintersection points is 0, 1, or 2 for distinct circles.
   We now prove that the intersection of two k -dimensional spheres is either the empty set, a ( k − 1)dimensional sphere, a k -dimensional sphere (which only occurs if the original spheres are identical andcoincident). Consider two spheres in k -dimensional space, and impose a coordinate system such that the centers of the two spheres lie on one coordinate axis. Then the equations for the two spheresbecome identical in all but one coordinate:
   2 2 2 2
   ( x − a ) + x + . . . + x = r
   1 1
   2 k 1
   2 2 2 2
   ( x − a ) + x + . . . + x = r
   1 2
   2 k 2
   General Test, Part 1
   If a = a , the spheres are concentric, and so they are either nonintersecting or coincident, intersecting
   1 2
   in a k -dimensional sphere. If a 6 = a , then subtracting the equations and solving for x yields
   1 2 1
   2 2 2 2
   r − a − r + a
   1 1 2 2
   x = . Plugging this in to either equation above yields a single equation equation that
   2( a − a )1
   2 2
   describes a ( k − 1)-dimensional sphere.
   Assume we are in dimension d , and suppose for induction that for all k less than d , any two distinctk -dimensional spheres intersecting in a finite number of points intersect in at most two points. Supposewe have a collection of d -dimensional spheres s , s , . . . , s . Without loss of generality, suppose the s
   1 2 m iare distinct. Let t be the intersection of s and s for 1 ≤ i < m . If any t are the empty set, theni i i +1 ithe intersection of the t is empty. None of the t is a d -dimensional sphere because the s are distinct.
   i i i
   Thus each of t , t , . . . , t is a ( d − 1)-dimensional sphere, and the intersection of all of them is the
   1 2 m − 1
   same as the intersection of the d -dimensional spheres. We can then apply the inductive hypothesisto find that t , . . . , t intersect in at most two points. Thus, by induction, a set of spheres in any
   1 m − 1
   dimension which intersect at only finitely many points intersect at at most two points.
   2009
   We now exhibit a set of 2 2010-dimensional spheres, and prove that their intersection contains
   √
   exactly two points. Take the spheres with radii 2013 and centers (0 , ± 1 , ± 1 , . . . , ± 1), where the signof each coordinate is independent from the sign of every other coordinate. Because of our choice ofradius, all these spheres pass through the points ( ± 2 , 0 , 0 , . . . 0). Then the intersection is the set of
   2 2
   points ( x , x , . . . , x ) which satisfy the equations x + ( x ± 1) + · · · + ( x ± 1) = 2013. The 2
   1 2 2010 2 2010
   only solutions to these equations are the points ( ± 2 , 0 , 0 , . . . , 0) (since ( x + 1) must be the same as 21
   2 i
   ( x − 1) for all i > 1, because we may hold all but one of the ± choices constant, and change theiremaining one).