Antiderivative of an even function

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Discussion Overview

The discussion revolves around the properties of the antiderivative of an even function, specifically whether the antiderivative retains the evenness or oddness of the original function. The context includes a mathematical exploration related to a partial differential equation (PDE) problem.

Discussion Character

  • Exploratory
  • Mathematical reasoning

Main Points Raised

  • One participant questions whether the antiderivative of an even function is also even, presenting a specific function defined by a definite integral.
  • Another participant suggests that the initial part of the question can be disregarded, citing sine and cosine as counterexamples.
  • One participant asserts that if the function is even, the intervals used in the definite integral cover mirror images of the real line, implying that the antiderivative may also be even.
  • A later reply provides a mathematical derivation showing that under certain transformations, the relationship between h(x,t) and h(-x,t) leads to a conclusion about their parity, indicating a potential oddness in the relationship.

Areas of Agreement / Disagreement

Participants express differing views on the implications of the antiderivative's properties, with some supporting the idea that it remains even while others introduce counterexamples and challenge the assumptions. The discussion does not reach a consensus.

Contextual Notes

The discussion includes assumptions about the nature of the function g and the specific intervals used in the integral, which may affect the conclusions drawn. The implications of the transformations applied in the derivation are also not fully resolved.

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Is the antiderivative of an even function even? Specifically, given an even function g(x)=g(-x) will the following function be even? Odd?

For constant a>0, r a dummy variable

h(x,t) = Definite Integral [ g(r) dr, FROM x-at TO x+at]

Asking whether or not h(x,t)=h(-x,t)


This is the last bit to a larger PDE problem and it's not coming to me...
 
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Ignore the first part. Sine/cosine is a simple counterexample. Still wondering about the second part.

Thanks
 
If the function is even, yes. [x-at,x+at] and [-x-at,-x+at] cover mirror images of the real line.
 
h(x,t) = \int_{x-at}^{x+at}g(r)\,dr

and g is even

h(x,-t) = \int_{x+at}^{x-at}g(r)\,dr = -h(x,t)

h(-x,-t) = \int_{-x+at}^{-x-at}g(r)\,dr

Let s = -r, ds = -dr

h(-x,-t) = -\int_{x-at}^{x+at}g(-s)\,ds = -h(x,t)

So

h(-x,t) = -h(-x,-t) = h(x,t)
 

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