Can I ask a professor (via e-mail) for a citation for a result presented in lecture notes? A mathematical result is presented in a set of lecture notes by a professor, and I have not been able to locate any journal papers which include the result. Would it be appropriate to ask the professor, via e-mail, for a citation to a journal paper?



Note: I am not a student of the lecturer.
Sure, it can't hurt to send a brief, polite email. Just make sure that the reference isn't already given somewhere in the notes, or the professor's course website, or his/her list of suggested reading.

Of course, you should prepare for the possibility that your email will go unanswered. Professors tend to be busy. There is little-to-no benefit to them (other than the pleasure of helping out another human being, that is) in tracking down a reference for someone who isn't a colleague, student, or potential collaborator. There's also the possibility that this professor didn't prepare his/her own lecture notes, and the grad student who did is long gone, etc.