Well, if I take the vector field that was in the problem set, and if I do things, say that I look at the unit circle.
如果找一个这个问题中的向量场，然后我们要做的是，假设这个单位圆。
3
Now there is a very famous problem with this procedure known as the Cartesian circle and I want to draw your attention to the fact that it's not only a problem for Descartes.