Uggh, now my brain hurts. :)
Actually, I needed to "solve" a problem for work. I researched textbooks, googled, Eric Weisstein, etc., but that Wikipedia artical came through for me.
"Solve" in quotes, because I actually had to unravel a somewhat botched analysis - the author, a Ph.D. physicist, could not explain his assumptions, he had relied on his own infallible intuition - and that article (and second edition Papoulis) provided me the key insights. Turns out, his intuition was plain wrong, but gave unfortunately plausible results.
They'll always be a tender spot in my heart for Wikipedia.