Sum of increasing and decreasing functions

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Discussion Overview

The discussion revolves around whether any real function can be expressed as the sum of an increasing function and a decreasing function. Participants explore the implications of such a representation, particularly in relation to the properties of the functions involved.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions if a real function can be expressed as the sum of an increasing function and a decreasing function, seeking clarification on the conditions required.
  • Another participant suggests considering the implications for the derivatives of the functions involved if such a representation is possible.
  • A different participant notes that the continuity and differentiability of the functions may affect the ability to construct such functions, indicating that piecewise continuous functions could satisfy the criteria under certain conditions.
  • It is mentioned that a function is the difference of two monotonic functions if and only if it is of bounded variation, referencing a specific mathematical resource for further exploration.

Areas of Agreement / Disagreement

Participants express varying views on the conditions under which the representation holds, indicating that the discussion remains unresolved with multiple competing perspectives on the topic.

Contextual Notes

Limitations include the dependence on the continuity and differentiability of the functions, as well as the specific definitions of increasing and decreasing functions. The discussion does not resolve these aspects.

chakib
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i want to know if any real function can be expressed as:
f(x)=g(x)+h(x) such as g(x) is an increasing function and h(x) is a decreasing function?
thanks
 
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Hello chakib, :welcome:

Here at PF we try to help folks to help themselves by (mostly) not providing direct answers, but providing help in the form of comments, hints, nudges, etc.

In this case: suppose you succeed, what does that mean for the derivatives of g and h ?
 
Hi those derivatives will tell you how the functions are changing, if that helps
 
The answer may depend on what limitations you place on f, g and h in terms of continuity and differentiability. If f is continuous I can almost trivially construct functions g and h that would satisfy your criteria but be only piecewise continuous.
 
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