Are These Logical Equations Equivalent?

AI Thread Summary
The discussion focuses on determining the logical equivalence of two equations: (∃x)(P(x) → Q(x)) and (∀x)P(x) → (∃x)Q(x). Participants are encouraged to explore the implications of these equations using alternative logical connectives and analogies, such as treating quantifiers as "and" and "or." The conversation emphasizes the use of deMorgan's Laws and the relationship between universal and existential quantifiers to analyze the statements. It suggests that if a biconditional between the two equations can be shown to be logically true, then the equations are equivalent; otherwise, they are not. Understanding these concepts is crucial for solving similar logical problems effectively.
axellerate
Messages
4
Reaction score
0
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)
 
Physics news on Phys.org
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.
 
I was reading documentation about the soundness and completeness of logic formal systems. Consider the following $$\vdash_S \phi$$ where ##S## is the proof-system making part the formal system and ##\phi## is a wff (well formed formula) of the formal language. Note the blank on left of the turnstile symbol ##\vdash_S##, as far as I can tell it actually represents the empty set. So what does it mean ? I guess it actually means ##\phi## is a theorem of the formal system, i.e. there is a...
Back
Top