Are These Logical Equations Equivalent?

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    Equivalence Testing
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SUMMARY

The discussion centers on determining the logical equivalence of two equations: (∃x)(P(x) → Q(x)) and (∀x)P(x) → (∃x)Q(x). Key insights include the application of de Morgan's Laws and the equivalence between universal and existential quantifiers. Participants emphasize the importance of understanding implications in terms of other logical connectives and suggest using biconditional truth to assess equivalence. The conversation highlights the necessity of formal proof techniques alongside intuitive reasoning.

PREREQUISITES
  • Understanding of logical quantifiers: universal (∀) and existential (∃).
  • Familiarity with logical connectives: implication (→), conjunction (∧), and disjunction (∨).
  • Knowledge of de Morgan's Laws in propositional logic.
  • Experience with biconditional statements and their truth conditions.
NEXT STEPS
  • Study the application of de Morgan's Laws in logical proofs.
  • Explore the concept of biconditional statements in depth.
  • Learn about formal proof techniques in mathematical logic.
  • Investigate the relationship between logical equivalence and truth tables.
USEFUL FOR

Students of logic, mathematicians, and anyone interested in understanding logical equivalence and proof techniques in formal reasoning.

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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