# What do you do if you find a typo in an equation of a paper?

I was reading an interesting paper, and early in the introduction there was an equation with a typo in it. I am absolutely sure of this, and all it was missing was a factor of $n!$. Overall, it was an inconsequential part of the paper, and doesn't affect anything, as it was in the section which tells the reader about the background of the area. But still, the equation as written is not correct, and could be confusing to someone reading who is trying to learn about the area.

In fact, the line right before this particular equation said "it is easily observed that..." which is a little interesting.

My question is: Should I do anything when I see this? Should I just ignore it, or is it polite to email the author? Or is it impolite to do so? If you published a paper, with an inconsequential, but possibly confusing typo in the introduction, would you want someone to tell you about it or just leave it alone?

Also please note, I am not referring to a spelling typo, that is definitely not important! Its just possible that this typo could confuse someone reading the paper. (I was confused at first!)

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I'll just note, as an author, I love to get emails about typos. It's so much easier than finding them myself. I've also gotten plenty of emails about mistakes in papers of varying severity, and I have always appreciated them very much (I can't say I loved getting them, but I'm very glad to have received them rather than not). –  Ben Webster May 14 '11 at 18:16
Beware of sentences like "it is easily observed that...". What they often mean is either "I have the argument but it's tedious and boring" or "I have the general idea, the details must be so simple that anyone could work it out in his spare time". Sentences of the second kind sometimes lead to actual mistakes (not typos). –  Felix Goldberg Apr 25 '12 at 8:43

Yes, it's appropriate to write the author and ask about this. Keep in mind that it's possible the paper is correct and you've misunderstood it... Mistakes of this sort are quite common, even in well respected textbooks and top journals.

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How would word it politely? I am still unsure about emailing, since it seems unimportant, but then again, possibly confusing. –  Eric Naslund May 14 '11 at 2:55
"I think there may be a typo in your paper. In equation X you're missing a factor of n!" The king thing in wording it politely is to say "may be" or "might" (rather than "is) and to say "typo" (rather than "mistake" or "error"). –  Noah Snyder May 14 '11 at 3:31
Or maybe "Dear Sirs, I have trouble understanding equation so-and-so of your paper. Might it be possible that there is a factor xxx missing in the equation?"... my point being, you're not trying to blame anybody, and you're taking into account the (more likely) possibility that you've misunderstood the equation in question. –  J. M. May 14 '11 at 3:51
What they said... –  Brian Borchers May 14 '11 at 3:59
Except don't start the email "Dear Sirs." You should be just fine with "Professor X." With most mathematicians, you can get away with first names, but especially as an undergrad there's no harm in starting off on a more polite note. –  Ben Webster May 14 '11 at 18:12

If I were the author, I would definitely prefer my paper to be error-free. In my opinion, pointing out a mistake isn't arrogant on your part, provided the mistake is genuine.

In order to avoid any friction, I think it is best to be completely honest in your wording of the mail when describing the error, and to make it clear why you think it would be confusing if left in the current uncorrected state.

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It's ironic that your first sentence has an error in it (at the time I am writing this). –  KConrad May 14 '11 at 4:31
I thought KConrad was talking about a very subtle error (if I was/if I were), but looking at the edit, I see there was more than one! –  Ben Webster May 14 '11 at 7:47
I was never good at English. (: –  Koundinya Vajjha May 14 '11 at 8:23
By the way, "If I was" is a subtle error in American English, but to the best of my knowledge quite correct in British English in which the subjunctive form is less common. –  J W May 14 '11 at 16:45
Is the BBC World Service (bbc.co.uk/worldservice/learningenglish/grammar/learnit/…) a British enough reference for you? Not that I really care; I wouldn't have noticed if I hadn't looked for an error in response to KConrad's comment. –  Ben Webster May 14 '11 at 18:09

I usually email the author in case the paper is online in a place controlled by the author (i. e., his website or arXiv). The only downside of this approach (apart from authors not replying) is that some would just take down their papers (rather than correcting them) when they hear about a mistake in a part they consider substantial. This happened to me once (although fortunately the paper is still avaliable at other places in the net).

In case the paper is online but not in such a place, this is a more difficult question, but if the error is substantially confusing, I'd still mail the author. Unfortunately, this sometimes means notifying a clueless author that somebody else has put his paper online (most usually, a university workmate, for his students; I am not talking about pirate sites), and rather than correcting it tries to get it offline. I would be careful here.

In case the paper is offline or behind a paywall, I ignore it unless the mistake really destroys some results from the paper, in which case I rejoice about another little blow to copyrighted literature and the so-called peer review process.

But then again, I am mostly reading openly avaliable texts, so the first case is by far the most frequent.

I, personally, would prefer anyone telling me of any mistake, but I do not have (and do not plan to have) papers outside of open access, so there is no contradiction here.

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Darij, concerning the last sentence: I wonder, if there is a time issue, i.e. when I find a typo or mistake in a paper that is, say, 20 years old. Then I fear the author was informed dozens of times about it and I'm uncertain if it's approriate to email him therefore. May I ask what's your opinion about it ? –  Ralph May 14 '11 at 9:29
I wouldn't care too much for old papers unless they are well-written and/or useful enough that they can serve as good learning material, in which case I'd notify both the author and the general audience (the latter by putting an errata list on the internet). –  darij grinberg May 14 '11 at 9:47