To solve this problem, we need to find the least positive integer k such that there exists a set T⊂S with more than 200 points where every pair of points in T are k-friends. This entails ensuring that for each pair of points A,B∈T, there exists a point C∈S such that the area of the triangle △ABC equals k.
Let's proceed with the solution step by step:
1. Understanding the Geometry:
- The area of a triangle △ABC formed by points A(x1,y1),B(x2,y2),C(x3,y3) is given by:
Area(△ABC)=21∣x1(y2−y3)+x2(y3−y1)+x3(y1−y2)∣
For the area to be k, we require:
∣x1(y2−y3)+x2(y3−y1)+x3(y1−y2)∣=2k
2. **Required Condition for k-friendship**:
- We want every pair of points A and B in the set T to be k-friends. This means for any two points, say (xi,yi) and (xj,yj), there should exist a point (xk,yk) such that the area of △ABC=k.
3. Ensuring Integer Area Values:
- The condition derived implies the determinant-like calculation must result in an integer. Hence, 2k should be a multiple of any determinant formed from integer coordinates.
- For any significant number of (xi,yi), the periodicity in area values can be ensured by the greatest common divisor (GCD) of these values being 1.
4. Using the Least Common Multiple (LCM):
- To ensure that every possible outcome for yi−yj results edges to 2k, we work with periods of such pairs.
- The smallest k that works should assure divisibility by each possible edge, i.e., k is a scalar multiple of the LCM of numbers up to a certain value dictated by the choice of over 200 elements.
- To sustain a large set, the determinant variations should be multiples of a common base scale horizon. This is physically by a required subgroup of grid coordinate segments. The complete LCM of the numbers from 1 to 14 provides such combinatorial grid guarantee up to 14.
5. **Calculating k**:
-
- Therefore, the minimum k can be computed as:
k=21lcm(1,2,…,14)
- Calculating this gives:
lcm(1,2,…,14)=360360
- Thus,
k=21×360360=180180
So, the least positive integer k for which there exists a k-clique with more than 200 elements is:
180180