added resources in mathematics and theoretical cs (#6129)

* Update free-programming-books-subjects.md

* Update free-programming-books-langs.md

* Update free-programming-books-subjects.md

* Update free-programming-books-subjects.md

* Update free-programming-books-subjects.md

* Update books/free-programming-books-langs.md
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>

* Update books/free-programming-books-subjects.md
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>

* Update books/free-programming-books-subjects.md
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>

* Update books/free-programming-books-subjects.md
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>

* Update free-programming-books-subjects.md

* Update free-programming-books-subjects.md

* Update free-programming-books-subjects.md

* Update free-programming-books-subjects.md

* Update free-programming-books-subjects.md
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>

books/free-programming-books-langs.md

@@ -500,6 +500,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [The Ultimate Question of Programming, Refactoring, and Everything](https://www.gitbook.com/book/alexastva/the-ultimate-question-of-programming-refactoring-/details)
 * [Think C++: How To Think Like a Computer Scientist](https://greenteapress.com/wp/think-c/) - Allen B. Downey (PDF)
 * [Thinking in C++, Second Edition, Vol. 1.](https://archive.org/details/TICPP2ndEdVolOne) - Bruce Eckel [(Vol. 2)](https://archive.org/details/TICPP2ndEdVolTwo)
+* [Working Draft, Standard for Programming Language C++, 2021 Revision](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/n4885.pdf) - Thomas Köppe (PDF) *(:construction: in process)*
 
 
 ### Chapel

books/free-programming-books-subjects.md

@@ -428,6 +428,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [OpenIntro Statistics](https://www.openintro.org/stat/textbook.php) - David M. Diez, Christopher D. Barr, Mine Çetinkaya-Rundel
 * [ORCCA: Open Resources for Community College Algebra](https://spaces.pcc.edu/pages/viewpage.action?pageId=52729944) - Portland Community College
 * [Ordinary Differential Equations](https://en.wikibooks.org/wiki/Ordinary_Differential_Equations) - Wikibooks
+* [Paul's Online Notes: Algebra, Calculus I-III and Differential Equations](https://tutorial.math.lamar.edu) - Paul Dawkins @ Lamar University
 * [Plane Geometry (1913)](http://djm.cc/library/Plane_Geometry_Wentworth_Smith_edited.pdf) - George Wentworth and David Eugene Smith (PDF)
 * [Planes and Spherical Trigonometry (1915)](http://djm.cc/library/Plane_Spherical_Trigonometry_Wentworth_Smith_edited_2.pdf) - George Wentworth and David Eugene Smith (PDF)
 * [Precalculus](https://stitz-zeager.com) - Carl Stitz and Jeff Zeager [(PDF)](https://stitz-zeager.com/szprecalculus07042013.pdf)
@@ -442,6 +443,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [Statistics Done Wrong](http://www.statisticsdonewrong.com) - Alex Reinhart
 * [SticiGui](https://www.stat.berkeley.edu/~stark/SticiGui/) - Philip Stark
 * [Tea Time Numerical Analysis](https://lqbrin.github.io/tea-time-numerical/) - Leon Q. Brin
+* [The Open Logic Text](https://builds.openlogicproject.org/open-logic-complete.pdf) - Open Logic Project (PDF)
 * [Think Bayes: Bayesian Statistics Made Simple](http://www.greenteapress.com/thinkbayes/) - Allen B. Downey
 * [Think Stats: Probability and Statistics for Programmers](http://greenteapress.com/thinkstats/) - Allen B. Downey (using Python)
 * [Vector Calculus](http://www.mecmath.net) - Michael Corral
@@ -573,6 +575,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [Computer Science from the Bottom Up](http://www.bottomupcs.com)
 * [How to Make a Computer Operating System](https://github.com/SamyPesse/How-to-Make-a-Computer-Operating-System) (:construction: *in process*)
 * [How to write a simple operating system in assembly language](http://mikeos.sourceforge.net/write-your-own-os.html) - Mike Saunders (HTML)
+* [Linux From Scratch](https://www.linuxfromscratch.org/lfs/view/stable/) - Gerard Beekmans, Bruce Dubbs, Ken Moffat, Pierre Labastie et al. (HTML, [PDF, downloads...](https://www.linuxfromscratch.org/lfs/downloads/stable/))
 * [Operating Systems and Middleware](https://gustavus.edu/mcs/max/os-book/) (PDF and LaTeX)
 * [Operating Systems: Three Easy Pieces](http://pages.cs.wisc.edu/~remzi/OSTEP/) (PDF)
 * [Practical File System Design: The Be File System](http://www.nobius.org/~dbg/practical-file-system-design.pdf) - Dominic Giampaolo (PDF)
@@ -724,6 +727,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Building Blocks for Theoretical Computer Science](http://mfleck.cs.illinois.edu/building-blocks/index.html) - Margaret M. Fleck
 * [Category Theory for Computing Science](http://www.tac.mta.ca/tac/reprints/articles/22/tr22.pdf) (PDF)
 * [Category Theory for Programmers](https://github.com/hmemcpy/milewski-ctfp-pdf) - Bartosz Milewski (PDF)
+* [Delftse Foundations of Computation](https://textbooks.open.tudelft.nl/textbooks/catalog/book/13) - Stefan Hugtenburgand, Neil Yorke-Smith @ TU Delft Open (PDF)
 * [Design of a Programmer](https://www.smashwords.com/books/view/639609) - Prakash Hegade (PDF)
 * [Homotopy Type Theory: Univalent Foundations of Mathematics](http://homotopytypetheory.org/book/) (PDF)
 * [Introduction to Computer Science](http://www.cse.iitd.ernet.in/~suban/CSL102/) - Prof. Subhashis Banerjee, IIT Delhi