The price of university textbooks has risen dramatically in the last few decades. Students often pay more than hundred dollars for a calculus text. Even used textbooks are expensive. One response to the rise in textbook costs is the Open Educational Resources (OER) movement. The term OER describes learning, teaching and research materials in any format and medium that reside in the public domain or are under copyright that have been released under an open license (usually GNU Free Documentation License or Creative Commons), that permit no-cost access, re-use, re-purpose, adaptation and redistribution by others. There are many advantages to open source textbooks beside cost. Errors may be fixed quickly. And every student has access to a textbook on the first day of class.

The American Institute of Mathematics supports the Open Textbook Initiative, where textbooks are reviewed and recommended by an editorial board.

Both *Abstract Algebra: Theory and Applications* and *The Ordinary Differential Equation Project* are licensed with a GNU Free Documentation License (GFDL). There is no cost to acquire these textbooks, and you are under no obligation whatsoever to compensate or donate to the authors or publisher—for all intents and purposes these are free textbooks. You may make as many copies as you like, ensuring that the book will never go out-of-print. You may modify copies of the book for your own use—for example, you may wish to change to a preferred notation for certain objects or add a few new sections. There is a copyright on the book, and subsequently it is licensed with a GNU Free Documentation License (GFDL), allowing the author to give you greater freedoms in how you use the text. The main caveat is that if you make modifications and then distribute a modified version, you are required to again apply the GFDL license to the result so that others may benefit from your modifications.

If a textbook is written in PreTeXt, a lightweight XML application, the textbook can be produced in different formats such as print, HTML, and even braille. Furthermore, any textbook written in PreTeXt and viewed in HTML format is fully accessible. Both *Abstract Algebra: Theory and Applications* and *The Ordinary Differential Equations Project* are written in PreTeXt. In addition, textbooks written in PreTeXt can be hosted on a Learning Engineering and Analytics Portal (LEAP) such as Runestone Academy.