2023 MPfG Olympiad Problem 4
by The_Turtle, Dec 5, 2023, 1:38 AM
Original 2023 Math Prize for Girls Olympiad Problem 4 wrote:
Let
and call a lattice triangle
marine if
. Find all points in
that don't lie in the interior of any marine triangle.


![$[ABO]=[BCO]=[CAO]=\frac{1}{2}$](http://latex.artofproblemsolving.com/a/8/c/a8c32d5f95cae4222c76f62d5c24dac5327c65f5.png)

Here is the original solution:
Solution
The answer is
. All such points cannot lie inside any marine triangle because some side
of any lattice triangle
containing
must intersect the ray starting at
pointing away from
. This forces
to contain
so by Pick's theorem,
Hence
cannot be marine.
![\[\sim\]](//latex.artofproblemsolving.com/c/d/1/cd1143692f2d3443a2baccf54c53881c3a1ef47c.png)
To prove that all other points are contained in some marine triangle, note that the transformations
preserve the marine-ness of triangles since shear transformations preserve area. Therefore
lies in a marine triangle if its image under one of the above transformations lies in a marine triangle.
Assume
has nonnegative coordinates; the other cases follow by symmetry. By repeatedly subtracting the smaller coordinate from the larger coordinate,
can be mapped to a point with sufficiently small positive coordinates, or to the point
if
for some
. If
then the point lies inside the marine triangle with vertices
as desired.








![\[[ABO] \geq \min\bigg(\underbrace{1 + \frac{1}{2} \cdot 3 - 1}_{\text{in interior}}, \: \underbrace{0 + \frac{1}{2} \cdot 4 - 1}_{\text{on perimeter}}\bigg) > \frac{1}{2}.\]](http://latex.artofproblemsolving.com/c/f/4/cf421248e82cdc6f0970ab9e57155b282e19737f.png)

![\[\sim\]](http://latex.artofproblemsolving.com/c/d/1/cd1143692f2d3443a2baccf54c53881c3a1ef47c.png)
To prove that all other points are contained in some marine triangle, note that the transformations
![\[(x, y) \mapsto (x-y, y) \quad \text{and} \quad (x, y) \mapsto (x, y-x)\]](http://latex.artofproblemsolving.com/9/3/c/93c50974ffbf98c97f758dadd05904f5a39c3ed8.png)

Assume






