Update free-programming-books-subjects.md (#7170)

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Co-authored-by: ImVector <59611597+LuigiImVector@users.noreply.github.com>

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Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

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

@@ -535,6 +535,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 ### Mathematics For Computer Science
 
 * [A Mathematical Theory of Communication](https://archive.org/details/bstj27-4-623) - Claude E.Shannon
+* [Combinatorial Problems And Exercises (1979)](https://archive.org/details/in.ernet.dli.2015.141538/) - L. Lovasz *(:card_file_box: archived)*
 * [Discrete Structures for Computer Science: Counting, Recursion, and Probability](http://cglab.ca/~michiel/DiscreteStructures/) - Michiel Smid
 * [Exploring Math for Programmers and Data Scientists](https://freecontent.manning.com/free-ebook-exploring-math-for-programmers-and-data-scientists/) - Paul Orland
 * [Isomorphism -- Mathematics of Programming](https://github.com/liuxinyu95/unplugged) - Larry LIU Xinyu