There is a good brief article on quadratic reciprocity on Wikipedia, assuming that my reference to how fascinating this topic is hasn’t caused the server to be overloaded with a tsunami of link requests. Mathworld also covers it, of course. It really is neat stuff. Gauss certainly thought so, and you can’t argue with ol’ CFG (Neal Stephenson is one of the very few people to acknowledge the “Johann.”)
Rick: A literary titbit. We had an exchange about Tennyson’s “In Memoriam” a few weeks ago. Did you know that when the poem was first published, anonymously, one reviewer hazarded a guess that the author was “the widow of a military man”?