Changing the order of integration: surefire method?

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Discussion Overview

The discussion revolves around the transformation of limits of integration for multivariable integrals without relying on geometric manipulations. Participants explore methods for determining new limits for integrals when changing the order of integration, particularly in higher dimensions where visual representations may not be feasible.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant inquires about algorithms or methods to transform limits of integration without using geometric illustrations, specifically asking how to determine functions like x_3(y,z) and x_4(y,z).
  • Another participant suggests that the region of integration can be defined by a system of inequalities and that these inequalities can be manipulated to find the desired form for the new integral.
  • A different participant expresses skepticism about the existence of a simple general algorithm, noting that the need to split the integral into several regions may arise if certain functions are not one-to-one.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the existence of a straightforward algorithm for transforming limits of integration, with some suggesting formal manipulations of inequalities and others highlighting potential complications in certain cases.

Contextual Notes

The discussion acknowledges that the transformation of limits may depend on the specific characteristics of the functions involved and the nature of the region of integration, which may not always allow for a simple solution.

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Is there a way to transform the limits of integration for a multivariable integral without appealing to geometrical manipulations? For example:
[tex]\int_a^b \int_{y_1(x)}^{y_2(x)} \int_{z_1(x,y)}^{z_2(x,y)} f(x,y,z) \; dz \; dy\; dx \rightarrow \int_c^d \int_{y_3(z)}^{y_4(z)} \int_{x_3(y,z)}^{x_4(y,z)} f(x,y,z) \; dx \; dy \; dz[/tex]
But how does one determine the limits for the second integral (most importantly, the functions [tex]x_3(y,z)[/tex] & [tex]x_4(y,z)[/tex], etc.) without actually resorting to an illustration of the function and its integration subset? Clearly illustrations are of no use in higher dimensions. Are there any algorithms that generate these interval transformations?P.S: For simplicity, let us assume that [tex]f(x,y,z)[/tex] is continuous, bounded, and generally "nice".
 
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The region of integration is simply the set defined by the system of inequalities

[tex]a \leq x \leq b,[/tex]
[tex]y_1(x) \leq y \leq y_2(x),[/tex]
[tex]z_1(x, y) \leq z \leq z_2(x, y).[/tex]

So, all you need to do is to formally manipulate these inequalities to put them into the form you want in the new integral.

I.E. You would first solve this system of inequalities for z, then for y in terms of z, then for x.
 
I would not expect a simple general algorithm since you may be forced to split the integral into several regions: That can happen say when one of the z_i(x,y),y_j(x) functions is not one-to-one.
 
Thanks Hurkyl and gammamcc.
 

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