Subtracting the overlap of functions

Click For Summary

Discussion Overview

The discussion revolves around the mathematical problem of subtracting one function from another specifically in the regions where the two functions overlap. Participants explore various approaches and considerations related to this concept, including the implications of non-overlapping regions and the behavior of the functions involved.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Conceptual clarification

Main Points Raised

  • One participant seeks guidance on how to subtract one function from another only in overlapping regions, expressing uncertainty about the approach.
  • Another participant suggests that subtraction can be performed directly in overlapping regions, as the functions are non-zero there, while noting that in non-overlapping regions, at least one function is zero.
  • A participant acknowledges the simplicity of the initial suggestion but raises a concern about the implications of calculating the subtraction in regions where one function is zero, indicating that a modification may be necessary.
  • One participant expresses a desire to restrict the functions to non-negative values before performing the subtraction and questions whether this restriction is specific to the functions being used.
  • A later reply proposes defining a characteristic function to represent the overlapping region, suggesting a method to isolate the subtraction to that area.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best approach to handle the subtraction in non-overlapping regions, and multiple views on how to manage the functions and their restrictions are presented.

Contextual Notes

Some participants mention the need for modifications when functions do not overlap, and there are discussions about the implications of restricting functions to non-negative values, but these points remain unresolved.

2thumbsGuy
Messages
24
Reaction score
0
I have a fun project I'm trying to do and it's been a good number of years since I did any math higher than algebra. As such, I don't know how to approach this and would like some pointers.

I am trying to understand how I can subtract one function from another ONLY where the two functions overlap. The first part is easy enough (algebra), but the caveat is challenging to my grey matter. Can anyone advise me on how to approach this?

Thank you,

2thumbs
 
Physics news on Phys.org
If two functions overlap in some region, it means they're both non-zero in that region. And in the regions where there is no overlap between them, it means at least one of them is zero in that region.So when you write f(x)-g(x), it only changes the value of the functions in the regions that they have overlap and so you don't have to do anything special, just subtract them!
 
I didn't think it would be that simple... let me just do that and see what I come up with. Thanks!
 
2thumbsGuy said:
I didn't think it would be that simple... let me just do that and see what I come up with. Thanks!
Just one more point. If you write f(x)-g(x), and then calculate it in a region where f(x)=0, although there is no overlap, g(x) will change because it gets multiplied by a minus sign. So a little modification to a simple subtraction is needed.
 
OK, I find it to be desirable where f(x) >= 0 and g(x) >= 0, so how can I restrict the function to that range before performing the subtraction? Or is it specific to the function?

EDIT

Here are the graphs I came up with to evaluate your statement:

Graph 1 ( f(x) )
http://www.wolframalpha.com/input/?i=-x^4+3x

Graph 2 ( g(x) )
http://www.wolframalpha.com/input/?i=-x^2+4x

Graph 3 ( g(x) - f(x) )

You can see on graph 3 the function at the right goes happily off to infinity on the y axis. Obviously this is because f(x) goes off to negative infinity faster than g(x) goes to negative infinity, so when inverted, f(x) wins. But if I stop them at zero before the subtraction, then I get my tidy little graph... I think
 
Last edited:
2thumbsGuy said:
I have a fun project I'm trying to do and it's been a good number of years since I did any math higher than algebra. As such, I don't know how to approach this and would like some pointers.

I am trying to understand how I can subtract one function from another ONLY where the two functions overlap. The first part is easy enough (algebra), but the caveat is challenging to my grey matter. Can anyone advise me on how to approach this?

Thank you,

2thumbs

It depends on what you are trying to do when they do not overlap. A sketch: Let A be the region where they overlap, define XA as 1 on A and 0 outside A. Then define f1 = f* XA and g1=g*XA. Now f1-g1 = f-g on A and 0 outside A.
 

Similar threads

  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 16 ·
Replies
16
Views
3K
  • · Replies 17 ·
Replies
17
Views
2K
  • · Replies 72 ·
3
Replies
72
Views
6K
  • · Replies 11 ·
Replies
11
Views
2K
  • · Replies 3 ·
Replies
3
Views
4K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 8 ·
Replies
8
Views
2K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 9 ·
Replies
9
Views
2K