Whether you are working toward your dissertation or a short research paper, managing citations and formatting them for your writing is getting easier all the time. There are several options for you to consider:
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There are a lot of free citation generators online, with varying levels of accuracy. ZoteroBib is one of the best. It uses the extensive database of the Zotero Reference Manager and is available free, ad-free, and with no account necessary. Supports over 10,000 citation styles.
Unlike other disciplines, mathematics doesn't have a single accepted way of citing sources. Ask your professor if they have a preferred bibliographic style before submitting a paper. Some faculty have no preference, as long as your sources are cited accurately. Chicago and AMS are two of the more common reference styles found in mathematics journals.
The Chicago Manual of Style offers two very different methods of citation: (1) notes-bibliography and (2) author-date.
While the notes-bibliography system is most commonly used in the humanities and the author-date system is most common in the sciences, you should always check with your instructor or publisher to find out which style you should use.
With American Mathematical Society (AMS) Style, in-text citations use a # sign in brackets to represent the order that the citation is mentioned in the text of the paper. For example, [5] would indicate that this is the fifth citation found in the text.