Maths joke (2)

A maths lecturer arrives a few minutes early for his lecture, and sees that there are only three students in the lecture hall, so he pops outside again to sit on a bench in the sunshine.

He watches as another two students go in and seven students come out. One of them comes up to him and asks him what he's waiting for.

"I'm waiting for the hall to fill up," he answers, "I'm not going to give a lecture to an empty hall."

They sit and watch as another two students walk in.

"There's an audience now," says the student. "Are you going to start the lecture?"

"No there isn't." replies the lecturer. "There's only you and me left, so I might as well go home."