# Course:Harris, Fall 08: Diary Week 13

From EscherMath

Mon:

- Exam II

Wed:

- Class generally did not do well on the "explain" problems, 3 and 6, of the exam.
- Students may gain extra credit (likely applied to the Exercises section of grades) by submitting corrections on those problems. Come see me before you submit.

- The Poincare disk has "ideal points": the points on the boundary (not really part of the hyperbolic model).
- Any two ideal points determine a unique geodesic that goes from one to the other.
- If P, Q, and R are ideal points (i.e., on the boundary), then the geodesics PQ and PR form a $ {0}^\circ $ angle at P.
- This can be seen by remembering that both PQ and PR must intersect the boundary at $ 90^\circ $.

- An ideal polygon is formed from geodesics (not just geodesic segments, but entire geodesics) which come together at ideal points.
- So all angles in ideal polygons are $ {0}^\circ $.
- So all ideal triangles have defect = $ 180^\circ $.
- That implies all ideal triangles have the same area, $ \pi $.

- Groups took 20 minutes to do the Hyperbolic Ideal Tessellations Exploration.

Fri:

- I mentioned that differences among the three geometries would be a primary theme of the end of the course.
- We used KaleidoTile to examine what happens when trying for regular tesselations of various Schlaefli types, {n,k}:
- In KaleidoTile, one can pick 3 numbers for "Choose Symmetries"
- The first is best left at 2 (I don't know what it does).
- If the point in the control triangle is very close to the left-hand corner, then
- the second number is n = # of sides in the regular polygon chosen for regular tessellation, and
- the third number is k = # of polygons around a vertex.

- We found that with n = 3 (triangles), we got
- for k = 3, it tesselates a sphere (tetrahedral)
- for k = 4, it tesselates a sphere (octohedral)
- for k = 5, it tesselates a sphere (icosohedral)
- for k = 6, it tesseallates the plane
- for k = 7 or higher, it tessellates hyperbolic space.
- We could tell that was hyperbolic because
- The triangles had inward-bending sides (i.e., angle-sums were less than in the plane).
- For a given line L and a point P not on L, there were multiple lines through P, all of them parallel to L.

- We could tell that was hyperbolic because

- In KaleidoTile, one can pick 3 numbers for "Choose Symmetries"
- Conclusion: Hyperbolic geometry can tolerate high numbers of regular polygons around a vertex, but not the plane or the sphere; the sphere takes the lowest number of regular polygons around a vertex.

- We used KaleidoTile to examine what happens when trying for regular tesselations of various Schlaefli types, {n,k}:
- Groups took 25 minutes to do the Three Geometries Exploration (needing some assistance with isometries).
- We largely omitted semi-regular tessellations.

- Next week, Monday will be devoted to trying to create with paper and tape that tessellation of 7 regular triangles around a vertex, thereby building ourselves a hyperbolic plane.