How to respond to intentional lack of citation? I work in a field that heavily uses arxiv.org and posts papers before submitting them to journals, so it's common for people to send a citation request when they see a preprint that they think has overlooked their work. I do this pretty rarely, because I think it's often abused by people looking to pad their citation count by asking for citations from work that's only marginally related. Still, when I see work that very closely resembles something I've done, I generally send an email.

My question is what to do when a paper appears that is very closely related to my work, has not cited me, and it was clearly intentional. In this case one of the authors had a previous paper that followed up on--and cited--one of my papers, so I know they were aware of my work in the past. Furthermore, in their current paper they use a technical term that I introduced. There is no possible way that they could have written their current paper without being aware of its close relationship to my work, but they haven't cited my papers.

In this context, the usual email of the form "I have read your interesting new paper and wanted to make you aware of my related work" would be so disingenuous as to be ridiculous: they are clearly already aware of my related work. I can only conclude that they have intentionally chosen not to cite it. All of my encounters with these authors in the past have been friendly, at least from my perspective, so I can't imagine any interpersonal conflict that's behind the omission.

A similar thing happened to me once before when some authors wrote a paper that had enormous overlap with one of my papers, which they cited, and they corrected, in passing, a minor technical mistake that didn't change any significant conclusions. Subsequently they built a minor industry on this work but never again referred to my previous work, with the result that their papers that were only a very minor improvement on mine have been cited several hundred times more than my paper has. So I may be overly touchy about this sort of possibility.

At the moment I'm inclined to do nothing, because any message I could imagine sending them would come across as either disingenuous or combative. But I wonder if anyone has a suggestion for dealing with this situation.
I don't know the details of your case, but in any case like this I tend to liberally apply Hanlon's Razor:


  Never attribute to malice that which is adequately explained by stupidity.


It does sound as if they know about your paper, but it seems to me entirely possible that they could have intended to cite it and accidentally omitted to actually do so.  I think you can contact them tactfully and point out that the original source for the technical term is your paper (give the complete reference).  They will probably reply with an apology for the oversight.  

If you want to be even more circumspect, you could ask a colleague in your subfield to contact them. ("I saw your preprint and it looks interesting.  The paper of Anonymous looks very relevant.  Do you think you should cite it?")

I've been guilty of such an accident myself.  I use BiBTeX to manage citations. I have a large file containing an entry for every paper I've ever considered citing.  Then for each paper I write, I include inline citations where they belong, and BiBTeX automatically picks the papers I've cited and generates the list of references.  In the case in question, I wanted to cite paper X in two places: in the introduction (as background) and in a specific technical passage.  I neglected to cite it in the introduction, but since it was also cited in the technical passage, paper X still appeared in the list of references, so when I saw it there I thought to myself "Good, I remembered to cite X."  Then in a later edit I changed the technical passage in such a way that X was no longer directly relevant there, so I removed that citation, thinking it was also cited elsewhere in the paper.  It wasn't, so BiBTeX accordingly removed it from the list of references, and it might certainly have looked to anyone as though I had intentionally failed to cite X (which was a well-known paper and clearly relevant).  The omission made it into a public preprint and was pointed out by a colleague (though not the author of paper X; he is long deceased), fortunately before the paper was submitted.

It was clearly my responsibility to have caught this and cited X correctly, and technical issues do not excuse my failure to do so, but I mention this as a specific example where such an error was due to stupidity rather than malice, and as evidence that the former could be an adequate explanation.