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PROJECT EULER · #0883

Remarkable Triangles

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In this problem we consider triangles drawn on a hexagonal lattice, where each lattice point in the plane has six neighbouring points equally spaced around it, all distance 1 away.

We call a triangle remarkable if

  • All three vertices and its incentre lie on lattice points
  • At least one of its angles is 60
0883_diagram.png

Above are four examples of remarkable triangles, with 60 angles illustrated in red. Triangles A and B have inradius 1; C has inradius 3; D has inradius 2.

Define T(r) to be the number of remarkable triangles with inradius r. Rotations and reflections, such as triangles A and B above, are counted separately; however direct translations are not. That is, the same triangle drawn in different positions of the lattice is only counted once.

You are given T(0.5)=2, T(2)=44, and T(10)=1302.

Find T(106).

Write-up coming later

The complete problem is available here. An approach, code, and answer will be added later.