Crunode
In mathematics, a crunode or node is a point where a curve intersects itself so that both branches of the curve have distinct tangent lines at the point of intersection. A crunode is also known as an ordinary double point.
For a plane curve, defined as the locus of points f = 0, where f is a smooth function of variables x and y ranging over the real numbers, a crunode of the curve is a singularity of the function f, where both partial derivatives and vanish. Further the Hessian matrix of second derivatives will have both positive and negative eigenvalues.
