Just out of curiosity:
What is the paper with the largest number of citations in all of mathematics?
I think it is Shannon's A Mathematical Theory of Communication (with its 71025 citations, according to Google Scholar), but I don't know any official sources about this.
What are other papers with a large number of citations?
(I am aware that the number of citations does not measure the importance of a work; this question is only for the sake of curiosity.)