Should I apologise to a professor for not citing his paper?

Question

Last year, I went to a math conference to present new results in my paper 1, which was just submitted to a journal. Afterwards, a well-known Professor X approached me to discuss the paper. He seemed very interested in the results, and mentioned paper 2, which was written by him and his collaborators. He didn’t ask me to cite his paper though (in hindsight, I guessed that he mentioned it just because he wanted it to be cited).

Recently, I asked him for a letter of recommendation. Initially, he agreed. However, after one month, he sent an email saying that he changed his mind, and criticized me for not citing his paper 2 in my paper 1, which is now published.

I already have enough references so his refusal doesn’t cause any harm. However, I wonder if I should apologise to him for not citing his paper?

The overlap of the two papers is as follows. My paper 1 has two main ingredients A and B, which are equally important. His paper 2 has two main ingredients A and C. I didn’t know about his paper until he told me, and thus I didn’t use any ideas or results of his paper.

Updates/Clarifications:

• When I talked about the main ingredients, I meant the overall paper, not the new results. Our paper didn’t have any overlap results. The part A was about the physical model (already well-established and in textbooks), which was our starting point. Our paths then diverged from this common physical model.
• I removed some references and an appendix to meet the page requirement, including this professor's paper and some other textbooks that described the physical model. We deliberately chose the least relevant ones to remove, and we didn't remove anything that would lead to plagiarism issues.
• As a side note, a senior colleague Y just showed me a paper 3, written by Y 5 years before paper 2, but Professor X didn't cite paper 3 in paper 2, even though they had similar mathematical results!

Answer 1

In general, you do not owe anyone an apology for not citing their work. Even assuming you were wrong not to cite the paper, that is essentially just a mistake in your assessment of the appropriate scope for your literature review; reasonable people can disagree on such matters (or just make mistakes), and it is not a personal snub against the authors whose works are not included.

It is possible that you were wrong not to cite this work, and some of your comments on your reasoning suggest that you may be operating under some misapprehensions in regard to this issue. In particular, your comment citing space limitations as the reason for non-citation is not a particularly good justification. (It sometimes only takes two words to cite a single-author paper in Harvard style, plus another one or two lines to add the paper in the reference list.) Similarly, while use of results in a paper is a good reason to cite it, it is not a necessary condition. Sometimes we cite a paper just to let the reader know that there is another work that examines the same/similar problem. In general, whether you cite a paper or not should be judged by whether or not it is helpful to the reader to know about the paper.

Giving a charitable interpretation to this professor's actions, one could say that he does not want to give you a letter of reference because he has concluded that you have poor judgment with regard to your ability to conduct a literature review. (The less charitable interpretation is that he conditions his assessments on whether or not the subject of the letter benefits his own citation metrics, which is a bit churlish.) That does not require you to make any apology, but you should take some time to self-reflect and consider whether this is a reasonable criticism of your work. You could certainly write back and thank the professor for taking the time to consider your work and offer a critique, and let him know that you will reflect on it. If you decide you made an error here (i.e., that you should have cited his paper) you could also write back and note your change of views, and in this case you could apologise if you wish, though it is not really necessary.

Answer 2

Should I apologise to a professor for not citing his paper?

The other two current answers are excellent, there are several good reasons to respond and at least acknowledge the professor's concerns and the relevance of their work (the "A" component). If it were me, I'd also include an explicit statement that you were not aware of the paper at the time and that your work was independent of it, to "allay any potential concern" that you might have not cited their work to make it "look like" you were not aware of it. Cover all of your bases in any paper trail.

However, after one month, he sent an email saying that he changed his mind, and criticized me for not citing his paper 2 in my paper 1, which is now published.

As mentioned elsewhere, we should strive to make the readers of our papers aware of all relevant/related existing work to the extent possible within practical constraints.

If it were me I'd include something like "In hindsight, I see your point and your paper would have been good to include along with the other citations" provided that you actually feel that way (never lie). You could even suggest that "Should I continue to publish in this area, I'll be sure to reconsider citing your work" and perhaps "Thanks for your advice".

It's a big world, but for any given topic of research, it's still a small world. Researchers of a given field constantly bump into each other, sometimes by reviewing each other's papers, and there are plenty of us with subconscious bias and a few that are even cut-throat and vindictive.

On the other hand...

You could also archive, print and frame the email and hang it on your wall. "I decided not to write you a letter of recommendation because you didn't cite my paper" (if that's what it really says) is a real gem! and anyone passing through your office and noticing it will get a chuckle! :-)

Answer 3

I should apologize to him for not citing his paper?

As a general rule, and not just in your specific case, I suggest erring on the side of apology without acknowledging wrongdoing. That is, do tell him: "I'm sorry I didn't cite your paper"; but unless you are sure you did something wrong, don't tell him: "I should have cited your paper".

Why?

• It will make the object of apology feel better.
• If you're not sorry for what you did, you can at least be sorry for having made him feel bad / slighted / disrespected.
• You're not actually giving him evidence for a potential formal complaint about your conduct. So you don't really have anything to lose.
• Apologizing does not mean losing face, or admitting a moral vice etc. So even symbolically you have nothing to lose.

Answer 4

Regardless of whether or not you should have cited the paper, in my opinion refusing to write you a letter based on this is a pretty clear overreaction. It's not like you plagiarized his work. In fact I think it borders on bullying behavior so you err on the side of citing him in the future. I wouldn't apologize. You can say something nice acknowledging his point if you want to avoid burning a bridge.

Answer 5

You owe him and yourself, if not an apology, at least an explanation why his paper does not deserve a credit.

The professor has criticized your actions, so you cannot ignore him at this point. Either, you apologize or you explain politely why you shouldn't cite his paper. Ignoring a legitimate comment by a scholar is a clear disrespect to the colleague, and is the worst kind of reaction.