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Gilbert Strang, Calculus, 3rd edition (Wellesley-Cambridge Press; MIT OpenCourseWare RES.18-001) §13.6 Maxima, Minima, and Saddle Points, #56 · Multivariable Calculus · Gradient & optimization

If …, show that … cannot have an interior maximum or minimum…

If $f_{xx} + f_{y y} = 0$ , show that $f (x, y)$ cannot have an interior maximum or minimum (only saddle points).

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Gilbert Strang, Calculus, 3rd edition (Wellesley-Cambridge Press; MIT OpenCourseWare RES.18-001) · §13.6 Maxima, Minima, and Saddle Points, #56

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