When including a reference in your bibliography, and the source has no publication date, how should this be indicated in the AMS (American Mathematical Society) citation style? Is it "n.d." or should the date simply be omitted?
I would say, yes,
n.d. is correct.
Background: The AMS Style Guide is based on the Chicago Manual of Style (see the Preface at p. ix), so let's take a look there (at chapter 17.119):
When the publication date of a printed work cannot be ascertained, the abbreviation n.d. takes the place of the year in the publication details. A guessed-at date may either be substituted (in brackets) or added. [...]
For the "guessed-at date", the Chicago manual provides the example of