Two functions, x2 and ex are walking down the street. Off in the distance they see a differential operator walking towards them. ex turns to x2 and tells him: "You better get out of here! If that operator differentiates you a few times you'll disappear, but he can differentiate me all he wants and I'll stay the same."
So x2 runs off and ex goes to greet the differential operator. "Hi, I'm ex" he says. "What's your name?"
The operator looks at him with a demonic grin and says: "d/dy".
* thanks to derick
Кстати... Сдала экзамен с радостью :)