PK Shiu June 6, 2004 observation Free Calculus Text PK Shiu June 6, 2004 observation This is from an old Slashdot review post, summarized the links here: First-Year Calculus Notes Difference Equations to Differential Equations: An Introduction to Calculus Lectures on Calculus Elementary Calculus: An Approach Using Infinitesimals The calculus bible