All of the material on this site is Free, with open source. In particular, this applies to the textbooks Linear Algebra, Theory of Computation, and Introduction to Proofs.
License
The material is Free.
Use it under either the
GNU Free Documentation License
or the 
Creative Commons Attribution-ShareAlike 3.0 United States License,
at your discretion.
For bookstores: thank you for being concerned with my rights. I give instructors permission to make copies of this material, either electronic or paper, and give or sell those to students.
I have tried to use only materials whose license allows me to share them. If I made a mistake and used something of yours that I shouldn't have then I apologize; please contact me.
Source code
If you are into LaTeX then you may be interested in my repository.
One use of the source is for instructors
to make small adjustments for their class, such as
adding an exercise or a special topic.
Feel free, but please also include a statement
so that people know who to write with bug reports.
Put something like this on the cover: \fbox{\parbox{0.75\textwidth}{The appendix on induction has been added by Professor Jones of UBU. For this material contact \url{sjones@example.com}.}}
Citations
I have been asked about citations for the books.
This website exists to be a
permanent home for links
so one like this is good:
Hefferon, Jim (2023).
Theory of Computation.
Open textbook.
https://hefferon.net/computation.
Contributions
If you have something to share, from a bug report to something bigger, then thank you, I'd be interested to see it.