==== gamma function ====
  * https://mathworld.wolfram.com/GammaFunction.html
    * contains all the important formulas, uses notation that is commonly used, content is easy to follow and understand, contains derivations of important formulas.

==== liate rule ====
  * https://en.wikipedia.org/wiki/Integration_by_parts#LIATE_rule
    * It is a rule of thumb to choose u when doing integration by parts.

==== tabular integration ====
  * https://en.wikipedia.org/wiki/Integration_by_parts#Tabular_integration_by_parts - shows how it works with a simple example
  * http://ramanujan.math.trinity.edu/rdaileda/teach/s18/m3357/parts.pdf - more rigorous, generic, has a lot of examples and addresses corner cases. Go through the wikipedia link before reading this.
    * Found this link in https://www.statisticshowto.com/tabular-integration-the-tabular-method/