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Is the best practice to number all equations, or just those that will be referred to?

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I prefer to number only referred equations. –  Marco Daniel Sep 23 '11 at 10:01
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I think this is highly debatable. I personally would number all equations, then a reader can reference equations you are not referring to. Also, if you happen to change your mind, you can later add a refernce more easily. And last, but not least, I find it typographically more consistent to have them all numbered. –  Count Zero Sep 23 '11 at 10:15
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This highly depends on the field of interest. In mine, displayed formulas are very seldom numbered and they're frequently given tags instead of numbers. I'd say that the choice depends also from the proportion between referenced and unreferenced equations. –  egreg Sep 23 '11 at 10:24

5 Answers 5

Use package mathtools. Then you define by an optional argument if only those equations should be numbered which are also referenced.

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Though, sadly has some bugs. –  daleif Sep 23 '11 at 11:35
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@daleif: I would be helpful to elaborate on the bugs you refer to, and perhaps provide links to other questions that discuss these bugs. –  Peter Grill Sep 23 '11 at 15:57

This depends on the field. However, in maths, engineering and natural sciences, it is preferable to number all references because it eases discussion.

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I agree, but also I think that in many cases there would probably exist some equations that are not worth numbering (e.g. intermediate results). –  nplatis Apr 17 '12 at 23:22
    
@nplatis: Numbers are cheap. Even in a massive book and by numbering all equations, numbers won't need more than three digits. So, the actual effort in numbering is small. –  thiton Apr 18 '12 at 6:52
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Numbers are indeed cheap and the author would need to think less if he numbers all equations. I think that non-numbered equations would stand out as non-important (if they are indeed). –  nplatis Apr 18 '12 at 10:14

I think there is something in numbering also equations which you do not refer to: if it is a publication to be used by other, they might want to cite your work with some specific reference to a particular equation. However, that might clutter your document quite a bit and produces large numbers ;) So my private convention is to number all equation except those which are inside a proof-environment. I guess this is quite fair, because if someone else wants to make reference, she can do so by referring to the lemma/theorem/etc which, of course, has a number.

Inside proofs, one has of course also the need to make references to earlier steps of the proof. But here I use \tag{$\ast$} instead of a number, which I understand to be a local numbering valid only inside the proof - there might by various equations inside proofs having the same symbol ($\ast$) but they are referred to only within the same proof. It also allows you to use nicer symbols for "local" equations like \smiley etc.

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According to Knuth et al:

Numbering all displayed formulas is usually a bad idea; number the important ones only.

and Halmos offers pretty much the same good advice,

What about "inequality (*)", or "equation (7)", or "formula (iii)"; should all displays be labelled or numbered? My answer is no. Reason: just as you shouldn't mention irrelevant assumptions or name irrelevant concepts, you also shouldn't attach irrelevant labels. Some small part of the reader's attention is attracted to the label, and some small part of his mind will wonder why the label is there. If there is a reason, then the wonder serves a healthy purpose by way of preparation, with no fuss, for a future reference to the same idea; if there is no reason, then the attention and the wonder were wasted.

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The text by halmos seems most applicable to a textbook or handbook. When publishing a scientific paper, it is less of my business to focus the reader's attention for him. If your ever tried to refer to a non-numbered equation in someone else's paper, you know what a hassle that is. So I wouldn't be too tight with numbers. –  Stephan Lehmke Mar 23 '12 at 9:42
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@StephanLehmke Halmos's advice is general and one should apply it to one's particular text. I think the message here, is don't go overboard with numbering and he same applies to the opposite. –  Yiannis Lazarides Mar 23 '12 at 10:01
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Whatever. I find it a bit paternalizing to place the author of a paper under the responsibility for where the reader puts their attention. Things which do not deserve attention should not be written at all, and above that, the reader may choose for themselves, that's my opinion. Writing for students is different, of course, because they need to be made aware what is most important. Again, my personal opinion. –  Stephan Lehmke Mar 23 '12 at 10:12

There are three conflicting rules about this practice:

  1. Fisher's rule. Number every equation.
  2. Occam's rule. Number only referenced equations.
  3. Fisher-Occam rule. Number equations that might be referenced.

These rules are described in "Writing in the Age of LaTeX" by Andrew D. Hwang in Notices of the American Mathematical Society (1995, issue 42) on page 881 as follows:

Mermin's first rule, "Fisher's Rule", is to number every displayed equation. His reason is that the author cannot know in advance if it will be necessary to refer to a specific equation or whether some future author (or the referee) may wish to do so. This is offered in contrast to "the heresy [called] Occam's Rule", which is only to number equations which are later referenced in the text, or the "Fisher-Occam Rule", which is to number equations which might be referenced later.

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