Are These Logical Equations Equivalent?

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  • Thread starter Thread starter axellerate
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    Equivalence Testing
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Discussion Overview

The discussion centers around the logical equivalence of two equations involving quantifiers and implications. Participants explore the thought processes and methods for determining whether the equations are equivalent, engaging with concepts from logic and formal proofs.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant presents two logical equations and seeks to understand their equivalence.
  • Another participant suggests rewriting the implications using other logical connectives as a potential method for analysis.
  • A further reply introduces an analogy to help conceptualize the quantifiers, likening universal quantifiers to "and" and existential quantifiers to "or," while referencing deMorgan's Laws.
  • Another participant mentions that determining logical equivalence can be approached by examining the biconditional of the two equations and checking for interpretations that could make it false.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the equivalence of the equations, and multiple approaches and interpretations are presented without resolution.

Contextual Notes

Some methods suggested depend on informal analogies and interpretations, which may not serve as formal proofs. The discussion includes various logical concepts that may require further clarification or formalization.

axellerate
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Hello hello, I'm not looking just for an answer per say, but am also wondering the thought process in solving problems such as the following:

Hopefully this doesn't take up too much of someones time.

Determine whether the following equations are logically equivalent:

1) (∃x)( P(x) → Q(x) )

2) (∀x)P(x) → (∃x)Q(x)
 
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Try to write the implication in terms of other connectives.
 
After you have followed micromass's suggestion, the next step can make more sense to you if you contemplate an analogy (I stress that this is a way of thinking: it would not work as a formal proof)
all quantifier like a large "and",
existence quantifier like a large "or"
"and" like "intersection"
"or" like "union"
deMorgan Laws.
Formally, if you are not an intuitionist, you can try playing around with the equivalence between "\forallx P" and "~\existsx ~P", or between "\existsx Q" and "~\forallx ~Q"

(by the way, it's "per se")
 
If you are familiar with how to determine a formula is logically true, then you can use the fact that formulas are logically equivalent just in case their biconditional is logically true. If there is an interpretation that makes the biconditional of (1) and (2) false, then they are not logically equivalent. If there is no such interpretation, then they are logically equivalent.
 

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