Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

If you have a formula with existential quantifiers, it is important in which order they appear.

Just to make an easy example:

$\forall$ man $\exists$ woman: the woman is the true love of the man

which is obviously a different statement than

$\exists$ woman $\forall$ man: the women is the true love of the man

The first one means that there a many women - eventually for every man another woman. The second statement means there is one women that is loved by all men. Good for the woman, eventually bad for the men.

If you have two existential quantifiers or two universal quantifiers, does the order make a difference?

share|cite|improve this question
4  
No, it does not. – Brian M. Scott Sep 23 '12 at 8:06
1  
$\forall$ man $\exists$ woman: some woman is the true love of the man -- I think it should be in this way. – Nikita Evseev Sep 23 '12 at 8:10
1  
Although, $\forall x \forall (y<x) : (y<x^2)$ certainly is dependent on order, since the other way round makes no sense! (even though it can be written as $\forall x \forall y : (y<x) \to (y<x^2)$ which is the same as $\forall y \forall x : (y<x) \to (y<x^2)$) – NeuroFuzzy Sep 23 '12 at 9:07
up vote 14 down vote accepted

Any number of successive quantifiers of the same kind can be replaced by a single quantifier by combining the quantified variables into a tuple; e.g. $\forall x\forall y$ is equivalent to $\forall(x,y)$. The order in the tuple is irrelevant, and thus so is the order of the quantifiers.

share|cite|improve this answer
1  
The question asks about ∀+∃ whereas you answer, accept and upvote mostly the answer to the different, ∀+∀ question. What is even more surprising is that the duplicate question has the same attitude, math.stackexchange.com/questions/491783/…! – Val Jan 9 '14 at 23:32
    
@Val Read the question more carefully - note the bolded part at the bottom. – Noah Schweber Jan 19 at 23:31

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.