It's a set of course notes from MIT 6.055 - "The Art of Approximation in Science and Engineering". They have been edited and updated into a book as well: https

It's a set of course notes from MIT 6.055 - "The Art of Approximation in Science and Engineering". They have been edited and updated into a book as well: https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-055j-the-art-of-approximation-in-science-and-engineering-spring-2008/readings/ https://ocw.mit.edu/courses/electrical-engineering-and-compu...