A Proving the Equivalence of Local and Global Maxima for Concave Functions

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The discussion centers on proving that local and global maxima coincide for concave functions, specifically through the theorem stating that a point is a maximum of the sum of two concave functions if and only if it is a maximum of a specific linear approximation of one of those functions. Participants suggest using the Intermediate Value Theorem and the Mean Value Theorem as potential tools for the proof. The conversation emphasizes the importance of formalizing the intuitive understanding of concave functions in relation to maxima. The request for assistance indicates a collaborative effort to clarify this mathematical concept. Overall, the thread highlights the relationship between local and global maxima in the context of concave functions.
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Proof of theorem. Intuition: local maxima and global maxima coincide for concave functions
Consider the following theorem:

Theorem: Let ##f## be a concave differentiable function and let ##g## be a concave function. Then: ##y \in argmax_{x} {f(x)+g(x)}## if and only if ##y \in argmax_{x} {f(y)+f'(y)(x-y)+g(x)}.##

The intuition is that local maxima and global maxima coincide for concave functions. But can anyone help me with a formal proof? Thanks in advance!
 
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Very interesting. Suggests me Intermediate Value Theorem and Mean Value Theorem, and this picture:
20211027_091904.jpg
 
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