The HNN extension, named after the mathematicians Graham Higman, B. H. Neumann, and Hanna Neumann, is a construction in group theory that allows the creation of new groups from existing ones. Specifically, an HNN extension is a type of group that is used to generalize the notion of groups with an additional structure, particularly when it comes to accommodating certain types of relations between groups.
What is the Ultraviolet Catastrophe? by Physics Explained (2020)
Source. It can be seen as the limit case of an Einstein solid at high temperatures. At lower temperatures, the heat capacity depends on temperature.
Wikipedia mentions that it is completely analogous to Planck's law.
youtu.be/Ca7c5B7Js18?t=803 compares Lagrangian mechanics equation vs the direct x/y coordinate equation.
Bdellium is a gum resin that is obtained from certain trees in the genus Commiphora, which are part of the Burseraceae family. The term "bdellium" is sometimes used to refer to a resin similar to myrrh. Historically, bdellium has been mentioned in ancient texts, including the Bible, where it is described as a valuable substance. The resin has been used for various purposes, including as an ingredient in perfumes, incense, and traditional medicine.
DokuWiki about physics, mostly/fully written by Jakob Schwichtenberg and therefore focusing on particle physics, although registration might be open to all.
"Seasons" typically refers to the four divisions of the year based on the tilt of the Earth's axis and its orbit around the Sun. These divisions—spring, summer, autumn (or fall), and winter—are characterized by changes in weather, daylight, and ecology. 1. **Spring**: Usually associated with warmer temperatures, blooming flowers, and new growth. It's often seen as a time of renewal and rejuvenation.
A nautical mile is a unit of measurement used primarily in maritime and aviation contexts to measure distances over the Earth's surface. One nautical mile is defined as exactly 1,852 meters (or approximately 1.15078 statute miles). The nautical mile is based on the circumference of the Earth and is equivalent to one minute of latitude.
Pinned article: Introduction to the OurBigBook Project
Welcome to the OurBigBook Project! Our goal is to create the perfect publishing platform for STEM subjects, and get university-level students to write the best free STEM tutorials ever.
Everyone is welcome to create an account and play with the site: ourbigbook.com/go/register. We belive that students themselves can write amazing tutorials, but teachers are welcome too. You can write about anything you want, it doesn't have to be STEM or even educational. Silly test content is very welcome and you won't be penalized in any way. Just keep it legal!
Intro to OurBigBook
. Source. We have two killer features:
- topics: topics group articles by different users with the same title, e.g. here is the topic for the "Fundamental Theorem of Calculus" ourbigbook.com/go/topic/fundamental-theorem-of-calculusArticles of different users are sorted by upvote within each article page. This feature is a bit like:
- a Wikipedia where each user can have their own version of each article
- a Q&A website like Stack Overflow, where multiple people can give their views on a given topic, and the best ones are sorted by upvote. Except you don't need to wait for someone to ask first, and any topic goes, no matter how narrow or broad
This feature makes it possible for readers to find better explanations of any topic created by other writers. And it allows writers to create an explanation in a place that readers might actually find it.
Figure 1. Screenshot of the "Derivative" topic page. View it live at: ourbigbook.com/go/topic/derivativeVideo 2. OurBigBook Web topics demo. Source. - local editing: you can store all your personal knowledge base content locally in a plaintext markup format that can be edited locally and published either:This way you can be sure that even if OurBigBook.com were to go down one day (which we have no plans to do as it is quite cheap to host!), your content will still be perfectly readable as a static site.
- to OurBigBook.com to get awesome multi-user features like topics and likes
- as HTML files to a static website, which you can host yourself for free on many external providers like GitHub Pages, and remain in full control
Figure 3. Visual Studio Code extension installation.
Figure 4. Visual Studio Code extension tree navigation.
Figure 5. Web editor. You can also edit articles on the Web editor without installing anything locally.Video 3. Edit locally and publish demo. Source. This shows editing OurBigBook Markup and publishing it using the Visual Studio Code extension.Video 4. OurBigBook Visual Studio Code extension editing and navigation demo. Source. 
- Infinitely deep tables of contents:
All our software is open source and hosted at: github.com/ourbigbook/ourbigbook
Further documentation can be found at: docs.ourbigbook.com
Feel free to reach our to us for any help or suggestions: docs.ourbigbook.com/#contact