What is the latin prefix for "2½ times" ?
- The question asks for the correct analogue of the prefix "sesqui-" which, of course, is the prefix for "1½ times".
This usage is antiquated and almost entirely replaced by the current meaning of a "serterce," which according to L&S is:
a small silver coin, originally equal to two and a half asses
L&S also quotes sestertium as meaning "two and a half feet [deep]," as used by Columella in De Arboribus:
ipsum agrum sat erit bipalio vertere: quod vocant rustici sestertium. (1)
...but this seems like sloppy research to me, since the sentence that immediately follows suggests a number more like 1 2/3:
...Ea repastinatio altitudinis habet plus sesquipedem, minus tamen quam duos pedes.
If you wish to use a prefix for "two and a half" then, you could feasibly go with sester-. It turns out that this phrase already has some traction, for instance as applied to a 250 year anniversary: "sestercentennial". See, for example, this New York Times article, this Wikipedia article, and ~28K Google results. These are all decidedly non-authoritative.
The above-cited Wikipedia article offers one other alternative that fits this case: semiquin-, which means "half of five."
The ultimate answer to your question is that there is no frequently used, easily understood prefix for "two and a half." This is probably because (!) it's not a terribly useful thing to have.
Classical attestations of any of the above suggestions are conspicuously absent, whereas we have from Varro a very clean example of what a Roman might have done when faced with the same question. Does he use "sesterpedis" to refer to a two-and-a-half-foot distance? Let's look:
...tympanum in latitudinem duo pedes et semipedem, in altitudinem palmum. (Varro, Res Rusticae 184.108.40.206)
So much easier to understand!
A table of numercal prefixes is given in Wikipedia; both cardinal & multiple forms. What we are duscussing here is the muliple form. Perhaps surprisingly, there is one for 1¼, which is quasqui. So perhaps one could use biquasqui for the 2½ multiple prefix.
But as for sester, I've also read of that one - I can't remember where now; but whatever the writing was that said it, it simply said without qualifcation it is the prefix, whereas a previous respondent speculates that it might be admissible. However, it is not in the Wikipedia table. I do use it and am going to continue to do so, anyway!
The previous respondent also says it's use-potential is borderline (not verbatim). As I said, I do use it; and I find that opportunities for doing so are not extremely rare. As for sestercentenial: an alternative there, as in the same table quater is given for the ¼ multiple prefix, would be quatermillenial.
There is perhaps no precedent for concatenation of bi & another prefix ... but concatenenation of it with any other of these multiple prefixes known to me would result in an elementary redundancy. It does so even in the case of quasqui if sester be admitted. And given quater, the concatenation hemisemi, as in hemisemiquaver in music, is similarly a redundancy.
As for the idea of 'half a syllable': I think that arises in connection with a word such as 'Popocatapetl', or 'Lyublyana': in the case of the former, a native Mexican will pronounce the '-etl' at the end as a single syllable, whereas an English speaker tends to make two syllables of it, but with some effort might be able to get it close to being a single syllable - perhaps a sesquisyllable; and likewise in the latter case - that of a native Slavonic speaker, & the 'Lyu-' & '-lya-' in 'Lyublyana'.