MathDB
2015-2016 Spring OMO #23

Source:

March 29, 2016
Online Math Open

Problem Statement

SS be the set of all 201722017^2 lattice points (x,y)(x,y) with x,y{0}{20,21,,22015}x,y\in \{0\}\cup\{2^{0},2^{1},\cdots,2^{2015}\}. A subset XSX\subseteq S is called BQ if it has the following properties:
(a) XX contains at least three points, no three of which are collinear. (b) One of the points in XX is (0,0)(0,0). (c) For any three distinct points A,B,CXA,B,C \in X, the orthocenter of ABC\triangle ABC is in XX. (d) The convex hull of XX contains at least one horizontal line segment.
Determine the number of BQ subsets of SS.
Proposed by Vincent Huang