Is there a y that does not exist for all x such that y^2 = x?

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Homework Help Overview

The discussion revolves around the logical negation of a mathematical statement involving the existence of a positive y for all positive x such that y squared equals x. Participants are exploring the implications of this statement within the context of real numbers.

Discussion Character

  • Conceptual clarification, Assumption checking, Mixed

Approaches and Questions Raised

  • Participants are attempting to formulate a useful negation of the original statement, with some suggesting alternative forms. Questions arise regarding the nature of counterexamples and what conditions would need to be satisfied for such examples to exist.

Discussion Status

The discussion is active, with various interpretations of the negation being explored. Some participants express confidence in the truth of the original statement while others are probing the conditions under which a counterexample could be considered.

Contextual Notes

There is an emphasis on the distinction between the existence of a counterexample and the truth of the original statement, particularly in the realm of real numbers. Participants are also navigating the implications of universal and existential quantifiers in their reasoning.

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Homework Statement



Give a useful negation for:

[itex]\forall x > 0, \exists y > 0 s.t. y^2 = x[/itex]

Homework Equations





The Attempt at a Solution



I'm not sure how to do this, I have

[itex]\exists y > 0 s.t. \forall x > 0, y^2 = x[/itex]

Which says "there exists a y that for all x, y^2 = x"...which is obviously incorrect, as there is no y that when squared equals EVERYTHING.
 
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How about, "there exists an x greater than zero such that for all y greater than zero, y squared is not equal to x"
 
If you were looking for an example to counter this (a counter example), what would need to be true for that example ? ... and, wouldn't it take just one example (at least as regards x)?
 
SammyS said:
If you were looking for an example to counter this (a counter example), what would need to be true for that example ? ... and, wouldn't it take just one example (at least as regards x)?

What do you mean counter it? I can't counter the first statement, it's true.
 
SammyS said:
If you were looking for an example to counter this (a counter example), what would need to be true for that example ? ... and, wouldn't it take just one example (at least as regards x)?

This supports my suggestion.
 
A negation of "for all x, this is true" is "there exists an x such that this is not true." You say "there exists an x such that for all (for any arbitrary) y this is not true" because it has to be not true for all y. if it's only not true for some y, then there exists a y such that it is true, and you've lost it.
 
1MileCrash said:
What do you mean counter it? I can't counter the first statement, it's true.
Well yes, it is true for x & y being real numbers. That doesn't mean that you can't coming up with criteria that would need to hold for a counter-example, if such existed.
 
Okay, I get it! So what you're saying, is that if there WERE a counter example, it would satisfy Arcana's negation statement, correct?
 
That's what I'm saying if indeed, Arcana's negation statement is correct.
 

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