Nondegenerate stationary point of a phase
= Nondegenerate stationary point of a phase
= Nondegenerate stationary points of a phase
{synonym}
A <stationary point> $x_0$ of a <smooth function>[smooth] phase $\phi$ is nondegenerate when its <Hessian matrix> at $x_0$ is an <invertible matrix>. In one dimension, this condition is $\phi''(x_0)\ne0$.