The famous professor of mathematics was in town for a conference.
Since he had some free time, he was approached to give a seminar for the undergraduate mathematics students at the local college.
After covering several blackboards with densely packed computations and expressions filled with Bessel functions and more, the professor remembered that there were many undergraduate students in the room.
Feeling just a twinge of remorse that perhaps he was talking above the heads of some of the students in his audience, he turned around and asked the audience if there were any students who had never seen a Bessel function.
The audience was silent for a moment
Finally, one intrepid student raised his hand to admit that he had never seen Bessel functions.
The professor nodded with apparent comprehension.
Without hesitation, he turned around and pointed at the blackboard, while saying “well, there's one now” and continued his talk.