How to map intervals of Real line

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Discussion Overview

The discussion revolves around methods for mapping intervals of the real line onto other intervals, specifically focusing on linear mappings. Participants explore general methods, provide examples, and discuss the applicability of these methods under various conditions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant asks for a general method to create a function that maps an interval (30, 140) to another interval (200, 260).
  • Another participant suggests using a linear map for this purpose.
  • A specific linear mapping function is proposed: f(x) = (6/11)(x-30) + 200, with a question about its general applicability for mapping any intervals (x1, x2) to (y1, y2).
  • A proof is provided to demonstrate that the proposed mapping function maintains the relationship between the intervals, ensuring that for any X in (x1, x2), the output f(X) lies within (y1, y2).
  • It is noted that while linear maps work in most cases, complications may arise when dealing with infinite or half-infinite intervals, suggesting the use of functions like exp() or tan() in such scenarios.

Areas of Agreement / Disagreement

Participants generally agree on the utility of linear mappings for finite intervals, but there is acknowledgment of potential complications with infinite intervals, indicating that multiple approaches may be necessary.

Contextual Notes

There is an assumption that the intervals discussed are finite, and the discussion does not resolve the specifics of how to handle infinite intervals or the implications of using alternative functions.

Who May Find This Useful

Readers interested in mathematical functions, interval mappings, and linear transformations may find this discussion relevant.

Diffy
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Can someone explain how to create a function that will map an interval of the real line onto some other interval?

Is there a general method?

Can you demonstrate? (30 140) to (200, 260)?

Thanks,

Diffy.
 
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try with a linear map.
 
Let x1 be in (30, 140) and let x2 be in (200, 260)
then f: x1 -> x2
f(x) = (6/11)(x-30) + 200

Something like that?

Will this always work for (x1, x2) -> (y1, y2)?

f(x) = (y2 - y1)/(x2 - x1) * (x - x1) + y1
 
Indeed, let us prove it as follws:

1. For any X so that x1<=X<=x2, we have y1<=f(X)<=y2
Proof:
We clearly have :
y1=(y2-y1)/(x2-x1)*(x1-x1)+y1<=f(X)<=(y2-y1)/(x2-x1)*(x2-x1)+y1=y2

2. Similarly, we can show that for any Y so that y1<=Y<=y2, then there exists some X, so that x1<=X<=x2 AND that f(X)=Y.

We get that X=(Y-y1)/(y2-y1)*(x2-x1)+x1. Since 0<(Y-y1)/(y2-y1)<1, that result follows immediately.
 
Thank you.
 
In most cases, you can use a linear map. Some trickiness results if one of the intervals is infinite or half-infinite. In that case, you might have to use exp(), tan(), or their inverses.
 

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