In mathematical literature "if and only if" (sometimes abbreviated as "iff"1) is a relatively common phrase. Saying "A if and only if B" means that A and B are equivalent logical statements. This is equivalent to but less clumsy than saying "A if B and A only if B". It is convenient to have conjunctions to say "A is implied by B" (if), "A implies B" (only if) and "A is equivalent to B" (if and only if).
I assume that this conjunction or phrase appears in Latin, too. To my surprise, I found no trace of it in this text of Gauss. When, if ever, did this phrase appear in Latin mathematical literature? And more importantly, what is this phrase in Latin? Of course, I could translate it as si et solum si, but I would like to know what was actually used.
Partial answers and non-mathematical contexts are also welcome if you have any insight to share.
1 The English abbreviation is "iff" (< if), the Finnish one is "joss" (< jos), the Italian one is "sse" (< se), the Spanish one is "ssi" (< si), the Swedish one is "omm" (< om), and so forth. By analogy, I would abbreviate the thing in Latin as ssi. But this question is about the fuller and more formal version.