When learning a new subject, I would frequently use lecture notes found somewhere in the Internet. When writing a paper (or a master thesis, as in my case, but the rules should be similar, I believe) one should give some reference for used results which are not common knowledge, if I understand correctly. This make me wonder: what do I do if I want to reference a result I found in some notes?

The natural thing to do would be to just add these notes to bibliography. What format would be preferable for this? Note that there will generally not be much publishing information, perhaps not even a definite year and place. (A BibTeX template would be .)

Secondly, is it OK to cite such materials as a reference?

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    I consider "lecture notes + internet link" (if this last exists) as perfectly good reference as long as the lecture notes are of good quality. – DonAntonio Jun 2 '13 at 19:36
  • Bibtex has tags like @unpublished, @misc, @online. – Shuhao Cao Jun 2 '13 at 19:37
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    "Secondly, is it OK to cite such materials as a reference?" Absolutely, but try to make sure the link will still be usable in a few years' time. (One approach is to ask the author via email to put them on arXiv.) Also, add the date of the publication to your reference, or it might become ambiguous. – darij grinberg Jun 2 '13 at 19:46
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    In mathematics, people cite unpublished papers whose preprints are not available or even personal communications. – user4511 Jun 3 '13 at 4:09
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    @Vahid: though in most cases when that happens, either the results are tangential or the results are reproduced in full. – Willie Wong Jun 3 '13 at 13:39

You should make a good-faith effort to find and cite original source of the results (to give proper credit). You should only cite the lecture notes if (1) they are the original source, or (2) the original source is inaccessible, either literally (out of print or unpublished) or figuratively (written in a foreign language, with excessive generality or formality, or just badly).

Finding the original source may require significantly more scholarly diligence on your part than the author of the lecture notes, since most lecture-note authors (including myself) are fairly sloppy with references. Such is life.

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    If the lecture notes contain the proofs in full, I don't see why one must find the original source. – Willie Wong Jun 3 '13 at 10:55
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    @WillieWong: surely it's good to give credit where credit is due. It is possible to over-do it, I suppose, but I think textbooks (not to mention lecture notes) err too much in neglect rather than over-footnoting. – paul garrett Jun 3 '13 at 13:01
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    @WillieWong: why not both? Not too burdensome to tell both a helpful source, as well as "origin" source. Delicate, yes, to say "don't try to read this, but cite it"? :) – paul garrett Jun 3 '13 at 13:17
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    @paulgarrett: I never said that one mustn't find the original source. I merely disagree with the notion that one must find the original source. If one can track down the original source, great! But having spent some time (out of personal interest) trying to track down, historically, the original instance of Widget X, I am inclined to be very, very relaxed about what constitutes a "good-faith effort". I especially will not demand a masters student to dig through the literature by himself for the first instance of Obscure Technical Lemma 3.1.52.... – Willie Wong Jun 3 '13 at 13:37
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    @WillieWong: Most of the scientific community agrees with you, to its detriment. – JeffE Jun 3 '13 at 14:02

To my taste, citations are fulfilling several purposes, some of which may not be fulfillable simultaneously. So, one should be honest about where one found a result, even if the source is not widely available. Thus, cite (in the best, most usable form possible) the lecture notes. Still, yes, accessible sources meet another criterion, namely, helping readers reproduce/understand your results.

Edit: in light of various comments and other answers... another purpose served by spending some (not unlimited) time finding original sources (even while being honest about the source one actually used or _learned_from_) is to give at least a lower bound for the age (and locale of origin) of the idea. Nevertheless, at the same time, it certainly can happen that a much later exposition does a much better job of explaining... after all, benefiting from hindsight.

Yet another reason to exert some effort to credit original sources is to dampen a bit a tendency that otherwise can dominate, namely, some form of "Great Man/Woman" syndrome, in which a very few people are portrayed as being responsible for nearly all good, big ideas.


From a general point of view, lecture notes are gray literature, meaning they might lack standard bibliographic metadata (you mentioned the year and place), may be harder to track down for readers, or not long-term available. Thus, one should generally prefer to cite conventional literature (such as books or articles in journals) over gray literature.

For a masters thesis, it should be fine to cite gray literature, but do check with your advisor. When you do so, you might as well discuss the format he'd recommend for citation. If you found the lecture notes online, one idea would be to cite it as online source, where key metadata would be the URL and the date of access.

In contrast to a masters thesis, many publishers discourage or forbid the citation of gray literature for journal papers. So if you want to make a paper from the thesis and the citation is essential, you would have to find the original source.

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