# correct typesetting for quantifiers

For years I have been typing and writing quantifiers in a certain way. Now that I am writing my thesis, my adviser is taking issue with some of these things. Since he is my adviser I’m going to do what he says, but I am curious about the general consensus on this.

As an example, let’s say I wanted to write symbolically “There exists an element $a$ of $A$ such that $a$ is positive.” My habitual way of doing this would be

$$\exists a\in A: a>0.$$

My adviser has 2 problems with this. Firstly he says there should be a space between the $\exists$ and the $a$. Secondly he says I cannot assume people will read the colon as “such that.” So he would have me change this to:

$$\exists\ a\in A\text{ such that }a>0.$$

Which seems correct to you?

As for the space after the $\exists$,
it looks funny to me.
It also seems significant that $\LaTeX$
does not automatically put a space after the $\exists$
and I need to write \exists\ instead of \exists.

As for the colon, it’s been a few years but I used to study logic, and I think in the conventions there my usage is fine. I’m not sure of the grammatical terminology but there is a sense in which the colon indicates that we are done quantifying things and are now going to indicate the property the quantified things have. Not only have I been writing this way for years, I have been teaching students to write this way.

I was never quite sure though about the colon with a universal quantifier? Like, is it conventional at least for some people, if I write “For all elements $a$ of $A$, $f(a)=c$” as follows?

$$\forall a\in A: f(a)=c.$$

A lot of people don’t write the universal quantifier at the beginning either. I feel that my more hardcore logic professors would never do this, but a topologist would have no problem writing:

$$f(a)=c, \forall a\in A.$$

The predicate logic conventions, if I’m remembering them properly, seem just way more … logical. But I want my writing to be familiar to my audience, which is something my adviser has the best feel for. I guess once I am more established I will have more freedom in how I write. In the meantime I’d like to hear which of these things look correct or incorrect, and please also mention what area of math you work in because that seems to matter.

First of all, I would avoid logical symbols as much as possible. For instance, you initial example could be phrased as “There exists a positive element $a$ in $A$”. Now, if you really need to use quantifiers, my advice would be the following:
2. Don’t hesitate to add spaces, and possibly parentheses and brackets to improve readability. $\LaTeX$ offers a large panel of possibilities to do so. For instance, $\exists x\ \forall y\ \varphi(x,y)$ looks better than $\exists x \forall y \varphi(x,y)$.
3. Put quantifiers in the front, not at the end. Although it is acceptable to write “$f(n) > 0$ holds for every integer $n$”, if you really need to use quantifiers, it is preferable to write
$$\forall n \in \mathbb{Z}\quad f(n) > 0$$
$$\forall n \in \mathbb{Z}\quad (f(n) > 0)$$