# Math 618

(Last updated Monday, April 19 at 1pm)

## Meetings

*The first class meeting will be on Monday, February 1 at 4:40pm. *

After the first week of class, we will use Monday's class time as an office hour meeting 4:40p–5:40pm and we will have a live class session on Wednesdays 4:40–5:40pm.

All meetings will occur on Zoom (Zoom Link).

## Materials

Contact: You can always reach me via email at nicholas.vlamis@qc.cuny.edu. I will generally respond within 24 hours.

## Week 11 (4/19--4/23)

Discuss calculating distance in the hyperbolic plane and prove that every Euclidean circle in the upper half plane is a hyperbolic circle. Prove that every hyperbolic circle is a Euclidean circle.Assignment 10 --- Due Friday, April 30

## Week 10 (4/12--4/16)

Introduce the notion of hyperbolic rigid motion and give some examples. Show that the inversion of a circle centered at the origin is a hyperbolic rigid motion.Assignment 9 --- Due Friday, April 23

## Week 9 (4/5--4/9)

On account of spring break and wanting to start hyperbolic geometry during a live session, there is only one pre-recorded video for this week.

Assignment 8 --- Due Friday, April 16

## Week 8 (3/22--3/26)

Prove that inversions negate angles between two lines, two circles, and a line and a circle.Prove a proposition about circles that we will use in class.Characterize the orthogonality of circles via action of inversions.## Week 7 (3/15--3/19)

Introduce inversions in circles.Prove a theorem characterizing the images of lines and circles under inversions in circles. Went over explicit examples computing images of lines and circles under inversions in a circle.## Week 6 (3/8--3/12)

Many students are having/had exams around this time, so I thought I would only post one video for this week. It will also allow us to make our transition to a new topic together in the live session.

## Week 5 (3/1--3/5)

Introduce translations and basic properties. Proved that every translation is the composition of two reflections about parallel lines. Introduced reflections. Skip Exercise 1: The definition of translation I gave in the notes is inadequate for guaranteeing translations are rigid motions. The easiest fix is to just assume from the beginning that translations are rigid motions (so in the definition I gave, simply replace the word “transformation” with “rigid motion” and all is good). With this definition, Exercise 1 on Assignment 4 no longer makes sense as a problem, so please skip it. Proved that a Euclidean rigid motion is the composition of two distinct reflections if and only if it is either a translation or rotation. ## Week 4 (2/22--2/26)

We show that rigid motions preserve lines and are completely determined by their behavior on any one triangle.Introduce reflections and prove that they are rigid motions. We proved that every Euclidean rigid motion is the composition of at most three reflections.## Week 3 (2/15--2/19)

*Schedule Change Notice:*The college is closed on Monday in observance of President's day. Office hours on Monday are cancelled, but Wednesday's class time will be treated as an office hour. There is only one video for this week, since we would have lost a day during a regular in-person class.

## Week 2 (2/8--2/12)

Introduce the notion of congruence and two congruence theorems (side-angle-side and side-side-side).Introduct the notion of parallelism and establish the Euclidean theorems about parallel lines that hold for all absolute geometries.Make sure to read the Homework Guide document so that you understand how to properly complete the homework.

## Week 1 (2/1--2/5)

Assignment 0 — Due Friday, February 5

*The Foundation's of Geometry*by David Hilbert (Of historical interest, not required)