I'm often writing equations with quantifiers like these:

x_i \quad \forall i = 1, \dotsc, n  

I've read this answer, but I'm not sure if my case is an example for a situation where one uses EM quad.

So my first question is, would you put a \quad space at that position?

The second question is, would you separate the first part from the quantifier with a comma like:

x_i \quad , \forall i = 1, \dotsc, n    
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    Personally, I'd avoid the \forall: x_{i},\qquad i=1,2,\dots,n I think that the quantifier is even mathematically wrong (at least in several cases I see). Note that you don't need \dotsc, because \dots is able to figure out what follows. \dotsc is needed only if you have an open ended enumeration, such as i=1,2,\dotsc – egreg Nov 20 '15 at 21:41
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    (1) I'd never leave a floating comma. (2) in most of what I edit I use a qquad for this leaving the single quad for "f quad and quad g qquad for all..." – daleif Nov 20 '15 at 21:42
  • Logically, \forall seems wrong. I don't know if mathematicians use it differently. Or don't remember well enough to be sure. But logically, it doesn't make sense. – cfr Nov 20 '15 at 22:52
  • @cfr assuming x_i is a boolean valued predicate, then it makes sense although would more normally be written with the quantifier first, \forall i \in \{1,\dots\n\} . x_i – David Carlisle Nov 20 '15 at 23:27
  • @DavidCarlisle It was more the = combined with the \forall which didn't make sense. (Although, logically, I'm inclined to see the i in x_i as unbound.) \forall i \in \{... for \text{for } i=1,\dots. It's the combination which doesn't look wff-like to me. – cfr Nov 20 '15 at 23:45

Don't put a space before the comma, Also I would not use \forall with an informal iterator displayed as dots.

Assuming x_i is some boolean valued expression indexed by i then either

x_i \quad \text{for all $i=1,\dots,n$}

or more formally

\forall i \in \{1,\dots,n\}\mathbin{.}x_i    

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