When I was a PhD student at Pisa, I received a handwritten letter. I read it, and for the very bad calligrafy jointly with the somewhat naive problems proposed, I asked me: "who wrote this letter?" When I decifred his signature I was very excited... Here's the letter:
A few weeks after, he answered to a letter of mine, and proposed me several new problems, one of which has been the subject of a joint paper, with Erdös and Deshouillers (see also papers).
The function P(x) quoted in the letter (one of latest letter of Paul Erdös) is the counting function of practical numbers.
For a lot of additional informations, photos, articles, and more and more you can go to the Erdös number project.