Page "Differential calculus" Paragraph 18
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If f is a differentiable function on R ( or an open interval ) and x is a local maximum or a local minimum of f, then the derivative of f at x is zero ; points where are called critical points or stationary points ( and the value of f at x is called a critical value ).
( The definition of a critical point is sometimes extended to include points where the derivative does not exist.
) Conversely, a critical point x of f can be analysed by considering the second derivative of f at x:
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