= Nonisolated stationary point
A <stationary point> is nonisolated if every neighbourhood contains other <stationary points>. For a twice <differentiable function>, a regular smooth curve of <stationary points> through a point forces its <Hessian matrix> to annihilate the curve's nonzero tangent: differentiate the vanishing <gradient> along the curve. Thus the <Hessian matrix> has a zero <eigenvalue> there, but that alone need not determine whether the point is a <local minimum> or <local maximum>.
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