Assume Einstein conjectured something (in Mathematics) and Zweistein conjectured something slightly stronger (implies Einstein's conjecture).

Which one of the following is correct?:

(a) Every X satisfies Zweistein's and thus Einstein's conjecture.
(b) Every X satisfies Zweistein's and thus Einstein's conjectures.

How about:

(a) Every X satisfies Zweistein's and Einstein's conjecture.
(b) Every X satisfies Zweistein's and Einstein's conjectures.

I went with (a), but was corrected by a native speaker. My feeling was that there are two separate identities and that "Zweistein's and Einstein's conjecture" is short for "Zweistein's conjecture and Einstein's conjecture".

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    Suppose we rewrite as "Every X satisfies the conjecture of Einstein and the conjecture of Zweistein". That is unambiguous. However as soon as you try to combine the two as "the conjectures of Einstein and Zweistein" it's impossible to recover the original info that it was one each. So my conclusion is that you should write it out in full, repeating conjecture. – user24964 Nov 15 '13 at 15:46
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    I would also use therefore instead of thus here. The two words are actually different and thus is really over used in scientific writing. Many people think it just sounds more official but it actually means in this manner rather than for this reason. – terdon Nov 15 '13 at 15:55
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    @terdon I disagree, therefore is used everywhere in math and science. Much more often than I see thus anyway – Cruncher Nov 15 '13 at 19:32

Your two pairs are not equal. The second should certainly be (b), since you are talking about satisying both conjectures instead of one or the other. But the first pair does not have quite the same meaning; it is not possible (as you explained, and presumably the audience know) to satisfy Zweistein's conjecture and not Einstein's. Therefore, the important thing is that Z's conjecture is satisfied. If follows that E's conjecture is also satisfied; but if you put 'conjectures' there, the reader might wonder whether E. has two relevant theories, both of which depend on Z.

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    So parentheses could be used to make the choice simple: (a') Every X satisfies Zweistein's (and thus Einstein's) conjecture. – Edwin Ashworth Nov 15 '13 at 15:42

One conjecture, made jointly by Zweistein and Einstein, equals "Zweistein's and Einstein's conjecture".

One conjecture made by Zweistein, plus one conjecture made by Einstein, equals "Zweistein's and Einstein's conjectures".

Since the second phrase is the one that matches your scenario, that's the one you need to use. (Essentially, because there are two separate identities, you need to use the plural "conjectures". :-) )

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    An alternative parsing of your second example is: "Multiple conjectures, made jointly by Zweistein and Einstein." – Andrew Coonce Nov 15 '13 at 20:28

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