= Critical points escaping to infinity in a Gaussian-weighted bilinear function
{title2=$f_\alpha(x,y)=xy e^{-\alpha(x^2+y^2)}$}
The origin is a <saddle point of a scalar function> for every real parameter. For $\alpha>0$ four further <critical points> have $x^2=y^2=1/(2\alpha)$. Equal-sign coordinates give global maxima and opposite-sign coordinates give <global minima>, with values $\pm1/(2e\alpha)$. The <Hessian matrix> there is $-4\alpha xy e^{-1}I$. Their distance from the origin is $\alpha^{-1/2}$, so as $\alpha\downarrow0$ they escape every compact region. At nonpositive $\alpha$ the origin is the only <critical point>. This illustrates why the number of <critical points> on an unbounded domain can change without a finite degenerate <critical point> appearing.
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