# X theorem or X's theorem

I am always stuck by the randomness of some syntax rules in academic articles, hence I cannot infer a proper answer to this question:

Is there a correct way to choose among "X theorem" and "X's theorem"?

I chose the word "theorem" for my concern comes from reading mathematics, but of course this questions stands for any reference to someone's work.

• Probably, "X's theorem" is the most correct form, but sometimes -- I guess -- the possessive "'s" is dropped for reasons of euphony. Compare, for instance, Ohm's law and Taylor series. Commented Dec 25, 2016 at 11:12
• I'd follow whatever is standard in the concrete case you're seeing. In my experience, the possessive form ('s) is typically used in English when you are talking of a result (e.g., a theorem, a law, a conjecture) as opposed to a mathematical object (e.g., a number sequence, a group, a function) or a whole subject (e.g., a theory) -- but there are exceptions (e.g., "Euler's number"). Also, the possessive form tends to be used only for single authors, because no one feels like saying "the Littlewood's and Richardson's rule". Commented Dec 25, 2016 at 12:33
• @AnonymousMathematician With the inconvenience that using the hyphen makes people believe that every hyphen joins two names: it took me years to realize that Lennard-Jones or Hanbury-Brown were not pairs of names but single people. Commented Dec 25, 2016 at 18:16
• @MassimoOrtolano: Good point! I can still remember thinking Birch, Swinnerton, and Dyer were three people. Commented Dec 25, 2016 at 18:19
• I say either. For example, "by the Dini theorem..." or "by Fejér's theorem..." Commented Dec 25, 2016 at 18:31

I am not a first-language English speaker, but a rule of thumb I have heard from several people (in mathematics) is: use exactly one among possessive and article. So: We compute the solution using Newton's method, or We compute the solution using the Newton method.

• "The Newton method" is not idiomatic in English.
– Buzz
Commented Dec 26, 2016 at 1:51

In health and medicine, there is a concerted effort to move away from possessive attributive eponyms. Thus, instead of Alzheimer's disease, it's now Alzheimer disease. The same is true of Parkinson disease, Gram stain, Cushing syndrome, etc.

• instead of Alzheimer's disease, it's *not* Alzheimer disease - I guess this should be it's Alzheimer disease Commented Dec 25, 2016 at 13:48
• Yes, you're correct, @morxa. Thank you for pointing out the error.
– user65587
Commented Dec 25, 2016 at 21:33

It should be either "X's theorem" or "the X theorem," not just "X theorem". There is no single rule saying just one of those is always correct. Sometimes both are and sometimes just one is.

Eisenstein's criterion or the Eisenstein criterion, Mordell's conjecture or the Mordell conjecture, Chebotarev's density theorem or the Chebotarev density theorem are all standard English. You can go either way.

What qualifies as correct English is what people who speak (native) English deem to be correct English. There is no way to apply logic to the matter. The answer to your question is that you should write whatever the common convention is, which you find out by reading and talking to people in the field. It could go both ways (see above) or only one way is standard and you should just use that. For example, it is standard to refer to "the Weil conjectures". Nobody these days talks or writes about "Weil's conjectures" for the set of conjecture he made about varieties over finite fields.