Why does it write all that stuff about f(adsbygoogle = window.adsbygoogle || []).push({}); _{xy}^{2}? Isn't it unnecessary?

I mean, isn't the following true?

If in a point (a,b): f_{x}=f_{y}=0 and f_{xx}f_{yy}>0, then the partial derivatives must be either both negative or positive, and thus point (a,b) is a local minima or maxima. And if f_{xx}f_{yy}<0 => (a,b) is a saddlepoint.

Right? So why bring in the f_{xy}^{2}?

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# Weird theorem on critical points for multivariable functions

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