Compute the number of ways a non-self-intersecting concave quadrilateral can be drawn in the plane such that two of its vertices are and , and the other two vertices are two distinct lattice points with and .
Solution
We instead choose points with and with in the interior of the triangle formed by the other three points. Any selection of these four points may be connected to form a concave quadrilateral in precisely three ways. Apply Pick's theorem to this triangle. If is the count of interior points, and is the number of boundary lattice points, we have that the triangle's area is equal to Let's first compute the number of boundary lattice points on the segment from to , not counting . This is just . Similarly, there are boundary lattice points from to . Adjusting for the overcounting at , we have and thus which we notice is periodic in with period . That is, the count of boundary points does not change between choices and . We wanted to find the sum across all of , the number of interior points . Using casework on , the periodicity allows us to just check across points with , and then multiply the count by to get the sum of across the entire row of points. For , we always have . For , we have at and for . Using periodicity, this -coordinate has a total a total of For , we have at and , and at both and . Using periodicity, this -coordinate has a total of For , we have at and at . Using periodicity, this -coordinate has a total of Adding our cases, we have ways to choose the four points. Multiplying back by the number of ways to connect the quadrilateral gives an answer of .