Скачать или смотреть What are...the three geometries?

Goal.
I would like to tell you a bit about my favorite theorems, ideas or concepts in mathematics and why I like them so much.

This time.
What are...the three geometries? Or: Normal, fat and thin triangles.

Disclaimer.
Nobody is perfect, and I might have said something silly. If there is any doubt, then please check the references.

Slides.
http://www.dtubbenhauer.com/youtube.html

Thumbnail.
https://d2r55xnwy6nx47.cloudfront.net...

Three geometries.
https://mathstat.slu.edu/escher/index...

Shape of the universe.
https://en.wikipedia.org/wiki/Shape_o...
https://astronomy.com/magazine/news/2...

Euclidean geometry.
https://en.wikipedia.org/wiki/Euclide...
https://mathworld.wolfram.com/Euclide...
https://www.britannica.com/science/Eu...

Spherical geometry.
https://en.wikipedia.org/wiki/Spheric...
https://mathworld.wolfram.com/Spheric...
https://www.britannica.com/science/sp...

Hyperbolic geometry.
https://en.wikipedia.org/wiki/Hyperbo...
https://mathworld.wolfram.com/Hyperbo...
https://www.britannica.com/science/hy...

Non-euclidean geometry.
https://en.wikipedia.org/wiki/Non-Euc...
https://www.quantamagazine.org/what-i...

Parallel postulate.
https://en.wikipedia.org/wiki/Paralle...
https://blogs.scientificamerican.com/...

Mathematica.
https://demonstrations.wolfram.com/An...
https://demonstrations.wolfram.com/Tr...
https://demonstrations.wolfram.com/Tr...

Pictures used.
https://demonstrations.wolfram.com/An...
https://en.wikipedia.org/wiki/Spheric...
https://en.wikipedia.org/wiki/Hyperbo...
https://i.stack.imgur.com/EhYlU.png
https://people.astro.umass.edu/~myun/...
https://astronomy.com/sitefiles/resou...{32863A9E-1283-4978-B3D0-B63688409E67}

YouTube and co.
   • Non-Euclidean Geometry Explained - Hyperbo...