1. http://math.mit.edu/classes/18.013A/HTML/chapter26/section01.html 2. https://www.math.upenn.edu/~moose/240S2015/slides7-28.pdf 3. https://nptel.ac.in/courses/111107098/15 4. https://www.youtube.com/watch?v=TYYhBhF4biU 5. https://www.youtube.com/watch?v=o2kbrqQgzOE 6. https://www.youtube.com/watch?v=Qscs_AZTf7c