There is an essay by Poincar? on differential equations. I am reading a part of it on kinds of singular points in a curve referring to a function or to a differential equation. He tells us that there are four kinds of singular points: first, crests , which are points through which two curves defined by the equation pass, and only two. Here, the differential equation is such that, in the neighborhood of this point, the equation is going to define and going to cause two curves and only two to pass. The second type of singularity: knots, in which an infinity of curves defined by the equation come to intersect. The third type of singularity: foci , around which these curves turn while drawing closer to them in the form of a spiral. Finally, the fourth type of singularity: centers, around which curves appear in the form of a closed circle.