Proving One-to-One Correspondence b/w (-1, 7) and R

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Discussion Overview

The discussion revolves around finding an explicit one-to-one correspondence between the interval (-1, 7) and the set of real numbers R. Participants explore various approaches, including geometric and algebraic methods, to establish this correspondence.

Discussion Character

  • Exploratory
  • Mathematical reasoning

Main Points Raised

  • One participant requests assistance in proving the one-to-one correspondence between (-1, 7) and R.
  • Another participant suggests starting with a simpler problem involving the interval (0, 7) and positive real numbers.
  • A participant proposes two approaches: a geometric visualization of projecting a finite length curve onto an infinite line, and an algebraic method involving an invertible function.
  • One participant expresses difficulty in identifying the function and considers using a piecewise function for the mapping.
  • A later reply discusses the flexibility in choosing the interval and suggests a formula for mapping one interval to another, emphasizing the importance of exploring various functions.
  • Another participant encourages creative thinking and exploration in constructing solutions, noting that generating incorrect answers can be informative.

Areas of Agreement / Disagreement

Participants do not reach a consensus on a specific function or method for establishing the correspondence, and multiple approaches are discussed without resolution.

Contextual Notes

Participants mention the potential use of standard functions and geometric interpretations, but no specific assumptions or definitions are agreed upon.

Who May Find This Useful

This discussion may be useful for those interested in mathematical reasoning, particularly in the context of set theory and functions.

andy.c
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I need help proving this:

Find an explicit one to one correspondence between the interval (-1;7) and the real numbers R

Any ideas?
 
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Hi andy.c! Welcome to PF! :smile:

Try an easier one …

find an explicit one to one correspondence between the interval (0;7) and the positive real numbers . :wink:
 
You can go about this two ways.

Geometrically try to imagine a projecting a finite length curve onto an infinite line.

Algebraically can you think of a standard invertible function with finite interval domain and all reals as its range (or vis versa)?
 
That about as far as i got too. I just can't think of the function.
I let f:(-1;7) --> R
f(x)= and I think it has to be a peeswise function if x is in (-1;7) and for all x not in (-1;7) but I got stuck there. :)
 
andy.c said:
That about as far as i got too. I just can't think of the function.
I let f:(-1;7) --> R
f(x)= and I think it has to be a peeswise function if x is in (-1;7) and for all x not in (-1;7) but I got stuck there. :)

It doesn't matter which interval the function you are looking for maps to the real line.

You can always shift and scale from one interval to another.

f: [a,b] -> [c,d]
f(x) = (d-c)/(b-a) (x-a) + c
(note I divide by the input width, multiply by the output width, subtract the start value a from x and add it to the result.)

Do you have a table of standard function, exponential, logarithmic, trigonometric, and so on? Look for a standard function mapping any finite interval to the whole real line or vis versa.

Or from the geometric end. Can you think of any geometric way to point to infinity? Like the horizon if you're standing on a plane? Imagine a way to point uniquely to any number on the real number line as if it were there in front of you. Think about how you point. You don't have infinitely long fingers or arms so your act of pointing should be in some finite range. Is there a way to label that range with the numbers in your finite interval?

EDIT: One last point. There are a multitude of right answers... don't think about solving for "the answer" think about constructing any answer using your imagination. Remember that in that sort of problem generating wrong answers are just as helpful because where they fail shows you something important. So try, guess, explore.
 
Thanks a lot, that made it a lot easier.
 

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