Terminology conventions in mathematics I am writing a paper where I have more than one lemma (Lemma 1, Lemma 2, and Lemma 3) and when I cite them together I was wondering is it more appropriate to say, for example, 

because of Lemmas 1-3

or 

because of Lemmata 1-3

I found out yesterday that Lemmata is the plural form of lemma but I had never seen it before used. Which one is conventionally used in mathematics?
 A: I am guessing we never write Lemmata in the literature because no one knows that the actual plural is Lemmata and not Lemmas. Since English is a very internationalized language and most English speakers are not native speakers, it becomes a reflex to write Lemmas for most people and sometimes this reflex can easily push native speakers into doing the same mistake if they hear/read it everyday. 
I myself use Lemmas for the same reasons. It seems kind of pointless to correct this "English mistake" if it goes against mainstream use, because it's just gonna raise questions in the reader's head instead of making him/her understand you just wanted to write the word in its plural form.
Hope that helps,
A: At the time that "lemma" is admitted as a word in a particular language, it should be applied accordingly to pluralize in that language no matter what the ethymological Greek (or Latin) plural whatever it is (in French the plural of "lemme" is "lemmes" and so is in Spanish, with an S at the end of the word).
The difference can be many ways; for example in Italian "paparazzi" is plural and "paparazzo" is the corresponding singular but many people don't use this singular to refer just one paparazzo; in Spanish the plural of "hermosa mujer" is "hermosaS mujereS" (twice S) but in English the plural of "beautiful woman" is "beautiful women" (non at all S), etc. 
