When should an equation be numbered when writing a paper?

Solution 1:

The dominant philosophy in most mathematical writing is to only number equations that are referred back to within the paper/article/note/book, or to number particularly important equations.

In general, there are three conflicting trains of thought with respect to numbering of equations in a paper.

1. Fisher's Rule: Number every equation, every time.
2. Occam's Rule: Number only those equations which are referred back to.
3. Fisher-Occam Rule: Number those equations which might be referred back to.

These rules were the subject of the article Writing in the Age of LaTeX appearing in the 1995 Notices of the AMS. But even there, some of the reasons for Fisher's Rule haven't aged particularly well.

Solution 2:

I try to do what the other answerers say to do: only number those equations I refer to. But on looking at the OP's sample page, another reason to number equations occurs to me: other future writers might want to refer to some juicy equation of mine, and it would do them a service to set things up so they could write "According to Kimchilover's equation (17), blah blah".

Added, 18 July: For example, in the body of the paper I say,

This and that. Clearly $$A=B$$ and so the set $S$ is bounded, proving the theorem.

The equation I don't number is $A=B$, which is referred to only by the sentence it occurs in. By the standard rule it gets no equation number. If I had foreknowledge of how important my work will be, I might say something like this in the introduction of the paper, where the problem and methods are described:

Theorem 2 then follows from the application of our Lemma 1, the Krein-Milman theorem, and the simple-looking equation (17).

And then give my faux-humble formula its number (17).

Solution 3:

You should only number equations that you are going to refer back. Most people are not very consistent. But now there are LaTex packages that do exactly that. See this post.

Solution 4:

I think that this was best expressed by physicist David Mermin in his column What's Wrong with These Equations (Physics Today, 1989):

[...]Fisher's rule is for the benefit not of the author, but the reader.

For although you, dear author, may have no need to refer in your text to the equations you therefore left unnumbered, it is presumptuous to assume the same disposition in your readers. And though you may well have acquired the solipsistic habit of writing under the assumption that you will have no readers at all, you are wrong. There is always the referee. The referee may desire to make reference to equations that you did not. Beyond that, should fortune smile upon you and others actually have occasion to mention your analysis in papers of their own, they will not think the better of you for forcing them into such locutions as "the second equation after (3.21)" or "the third unnumbered equation from the top in the left-hand column on p.2485." Even should you solipsistically choose to publish in a journal both unrefereed and unread, you might subsequently desire (just for the record) to publish an erratum, the graceful flow of which could only be ensured if you had adhered to Fisher's rule in your original manuscript.

Solution 5:

The reason I think is because the first two equations serve as "steps" for the third one. And so it is not as important how they got the equation, rather then the equation itself.

It's like: find $z$ such that $z = x + y$ where $x=5, y= 6$.

$$x=5$$

$$y = 6$$

$$z = x + y = 11\text{ (1.2)}$$

$z$ is the most important here, not how we got it, and so in your case, the bound is the most important, rather then the thinking that went behind it. This is my guess. I'm not sure tbh.