Upper and Lower Derivatives of the Characteristic Function of Rationals?

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SUMMARY

The upper and lower derivatives of the characteristic function of rationals are defined based on the nature of the point being evaluated. At a rational point, the upper derivative is 0 and the lower derivative is negative infinity. Conversely, at an irrational point, the upper derivative is infinity while the lower derivative is 0. This distinction is crucial for understanding the behavior of the characteristic function in different contexts.

PREREQUISITES
  • Understanding of characteristic functions in probability theory
  • Knowledge of upper and lower derivatives in real analysis
  • Familiarity with rational and irrational numbers
  • Basic concepts of limits and continuity
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  • Study the properties of characteristic functions in probability theory
  • Explore the definitions and applications of upper and lower derivatives
  • Investigate the implications of rational versus irrational points in analysis
  • Learn about continuity and discontinuity in the context of characteristic functions
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Mathematics students, analysts studying characteristic functions, and anyone interested in the properties of derivatives in real analysis.

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Homework Statement



What are the upper derivative and lower derivative of the characteristic function of rationals?

Homework Equations





The Attempt at a Solution



I think they are : upper derivative = 0
lower derivative = negative infinity
 
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I agree that the lower derivative is 0. But I think the upper derivative depend on wether you take them in a rational or irrational point.
 
If the point is rational: then the upper derivative is 0 and the lower derivative is negative infinity. (as you postulated)

However, if the point is irrational, then the upper derivative is infinity, and the lower derivative is 0. (for pretty much the same reason as in the rational case, except that the signs are reversed)
 

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