Add Jean Gallier's Differential Calculus book

free-programming-books.md

@@ -754,6 +754,7 @@ See also [TeX](#tex)
 * [Mathematica® programming: an advanced introduction by Leonid Shifrin](http://www.mathprogramming-intro.org/)
 * [Stephen Wolfram's The Mathematica Book](http://reference.wolfram.com/legacy/v5_2/)
 * [Wolfram Mathematica Tutorial Collection](http://www.wolfram.com/learningcenter/tutorialcollection/)
+* [Basics of Algebra, Topology, and Differential Calculus](http://www.cis.upenn.edu/~jean/math-basics.pdf)
 
 
 ###Maven