# When shall one write a mathematical expression as a separate line in a paper/Ph.D. thesis? [duplicate]

When do we write a mathematical expression on a separate line, and when do we write it inline with the text?

I believe that the answer is the same for researching papers and Ph.D. theses, or maybe even textbooks; but please correct me if I am wrong.

Here I am referring to a paper/thesis on mathematics. When one writes a mathematical paper, sometimes one writes a mathematical expression in a separate line such as in the following example:

We know that the identity
a2+b2=c2
holds true if ....

While sometimes one writes the mathematical expression inline with the text, such as in the following example:

Let f:X->Y be a one-to-one map, ...

My impression is the longer the expression is, the more likely it is written in a separate line. But is there any (possibly unwritten) rule for it?

• I can remember that there was a question about when to use inline or display mode for equations on either Academia.SE or TeX.SE websites; I searched for it but I can not find it... Dec 22, 2014 at 8:57
• Don't make a mountain out of a molehill. Make sure your paper is clear and easy to follow. That's what matters. Dec 22, 2014 at 16:02
• Most people read more papers than they write. The papers you read should give you a good feel for what should, and should not, be displayed. Dec 23, 2014 at 1:55

In the areas in which I work, there are no formal rules and it is left up to the author when to separate an equation and when to embed it in the text. When I am writing, I find that there are three fairly straightforward principles that work well for me in determining whether an equation (or other mathematical statement) deserves its own line or can be inline with the text:

1. Will I need to refer to the equation elsewhere? If so, it needs its own line, and equation numbering as well.

2. Is it more than ~1/3 of a line long? Anything so long that it is likely to get broken across lines and otherwise be a typographical mess should be pulled out onto its own line. Numbering is not required.

3. Do I want the reader to "pause" and contemplate the equation, or do I want it to "flow" as part of the sentence encompassing it?

This last needs a little bit more explanation... let's elaborate on one of the examples from the original question. If I write the sentence this way:

We know that the identity a^2+b^2=c^2 holds true if the system is in condition X.

then the equation should be inline, because the sentence is really about condition X, rather than about the equation. If, on the other hand, the reader's attention should be directed to the equation, then it is better to use something like the following form, with the equation on its own line:

We know that if the system is in condition X, then the following identity holds:

a^2+b^2=c^2

Note: these principles reduce to a similar effect as the style guide given by @silvado

• When numbering equations, keep in mind that reviewers may want to refer to equations even if the author doesn't. Thus I'd recommend numbering most equations in display. Dec 22, 2014 at 14:02
• @silvado I find that it really depends on the type of paper. When the paper is math-centric, I completely agree with you. In certain cases when there are only a couple of equations in an otherwise entirely experimental paper, it may make less sense. Regarding reviewers: don't forget that many journals put line numbering on drafts, which means equation numbering superfluous for this purpose. Dec 22, 2014 at 14:10

The book "Handbook of writing for the mathematical sciences" by N. J. Higham shortly discusses that point in Section 3.7 ("Displaying equations"). I cite the first sentence of that section:

An equation is displayed when it needs to be numbered, when it would be hard to read if placed in-line, or when it merits special attention, perhaps because it contains the first occurrence of an important variable.

In all other cases, equations should be put inline (being a mathematician, the book author does not mention this explicitly).

You may want to check the book Mathematics into Type published by the American Mathematical Society (AMS). See particularly section 2.5 in the Updated version (1999).

There are basically no fixed rules for what should be typeset as so-called display (on a separate line) versus run into the text. Length of the equation as well as importance are key parameters in making such a decision.

Length. Long equations will be difficult to set and to read if set into the text. Hence they need to be set in display mode.

Importance. If an equation is important to the text then it is likely better to set in in display mode since it will be easily seen.

Running everything in display is not useful. in-text equations save space while display equation break up the text. From this it is evident that the mix should also consider the length of the final text and the readability of the text.

There may of course be specific instructions for individual journals so check, in your case earlier PhD theses for hints, perhaps with the book mentioned above in one hand.

As with any style question, work through these rules from the top, and stop when you have an answer: