Lately, I writing some essays whose topics are mathematics-heavy. Even though they are not research papers and will never be published, I just want to give proper references to each famous theorem/ideas.
However, finding the original source of each theorem proves to be a much more difficult task than I thought. This brings me to my question
How, in general, do I find the original papers of each famous theorem?
Specifically, how do I find Caratheodory's paper on the extension theory that now bare his name?