Problem in understanding the meaning

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Discussion Overview

The discussion revolves around the interpretation of two mathematical expressions involving the existence of values in relation to a function. Participants explore the implications of each expression, focusing on the uniqueness of the corresponding values.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants suggest that the first expression indicates that for every x value, there is a corresponding y value, implying a function.
  • Others argue that the second expression suggests there exists one or a set of y values for all x values, raising questions about whether it refers to a single y value or multiple values.
  • A participant asserts that "there exists y" typically means there is at least one y value, but does not specify uniqueness unless explicitly stated.
  • Another participant notes that the first expression does not define a function unless it specifies that y is unique, while the second expression implies that if such a y exists, it must be unique based on the definition of a function.

Areas of Agreement / Disagreement

Participants express differing interpretations of the implications of the two expressions, particularly regarding the uniqueness of y values. No consensus is reached on the exact nature of the relationship between x and y in the context of the expressions.

Contextual Notes

Participants highlight that without additional qualifiers, the expressions do not clarify whether y is unique. The discussion also reflects varying interpretations of mathematical notation and terminology.

thudda
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What is the difference between the 2 expressions

1) for all x belongs to ℝ there exists y belongs to ℝ such that f(x)=y
2) there exists y belongs to ℝ such that for all x belongs to ℝ , f(x)=y

I want to know the exact difference.
 
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hi thudda! welcome to pf! :wink:

tell us what you think, and then we'll comment! :smile:
 
Hi..thanks..:)

I think the 1st expression implies that for every x value there is a corresponding y value.And the 2nd imply that for all x values there's one or a set of y values...what I want to know is whether it is one y value or a set of y values..
 
hi thudda! :smile:
thudda said:
I think the 1st expression implies that for every x value there is a corresponding y value.

yup … basically, it doesn't say anything more than that f is a function! :wink:
2) there exists y belongs to ℝ such that for all x belongs to ℝ , f(x)=y
And the 2nd imply that for all x values there's one or a set of y values...what I want to know is whether it is one y value or a set of y values..

ah, it's one y value …

"there exists y" always means there exists a y :smile:

(btw, when i see ##y\in R##, i always read that as "y in R" … it's shorter, and i think, easier, than "y belongs to R" :wink:)
 
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tiny-tim said:
"there exists y" always means there exists a y :smile:

Without more, both "there exists a(n)" and its abbreviation [itex]\exists[/itex] mean "there exists at least one". If you want to specify uniqueness, you must do so expressly. The abbreviation for "there exists exactly one" is [itex]\exists ![/itex] - the usual symbol followed by an exclamation mark.

As to the OP's examples: The first isn't quite the definition of a function; it would be if it asserted that y was unique. In the second, it follows from the definition of a function that if such a y exists then it must be unique.
 

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