Soit A une partie de N

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Senior Member
English, Singlish, Chinese; Singapore
I saw this in the French Wikipedia and I was highly intrigued. It's in a mathematics context, ie. "let A be an element of set N". It seems funny as an imperative, because that would mean "[you should] be A, a part of N", while if the subjunctive (most likely is), it is being invoked without the "que".

I've also seen:

Supposons que (x1, x2, ...) soit une suite de nombres réels.
Is the subjunctive always used with "supposer que", or is it optional? I just thought its use was very cool from a mathematics perspective, it's a register of language that English can't match up to in the textbooks.
  • Jalc

    French - France
    "Soit" is indeed a strange animal here. I used it a lot...

    If you read in the Petit Larousse, they will explain it means "Etant donné", and I never really considered this as a verb. Nevertheless, for plural, you will use "Soient A et B, ...".

    But the imperative of "être" would be "Sois". Here it is the subjunctive conjugation. Your explanation of ommission of "Supposons que" is really interesting!


    New Member
    France/ Francais
    Je pense que la phrsase "supposons (ou peut etre posons) que A soit..." est l'explication correcte de l'emploi du subjonctif. C'est tres habituel dans les problemes de maths.


    Senior Member
    Hi. French conjugation tables often conjugate the subjunctive mood with "que" like "que je sois, que tu sois, qu'il soit".
    This is a very bad habit, because it makes believe that the subjunctive mood cannot occur without que, which is not true. I can give plenty of examples.
    Here, the subjetc of "soit" is A. it is a subjunctive, but acts almost as an imperative. When you say in the imperative : "sois gentil", you mean : "je veux que tu sois gentil" (subjunctive).
    in the same way, soit A means : "nous voulons que A soit".

    For "supposons que", I had noticed it when I was a student in mathematics. You usually say : "je suppose qu'il est", using the indicative.
    In the negative, you say : "Je ne suppose pas qu'il soit", using the subjunctive. I guess this is normal, since it acts as the verb "croire".

    In the imperative, in mathematical prooves, I had never found strange to encounter "supposons que A soit", with the subjunctive, although the sentence is NOT negative. One day, in a well written demonstration, I found "supposons que A est", which helped me notice the strange use of the subjunctive mood.
    I don't think there is a rule that compels to use the subjunctive in an imperative statement, so my guess was that "supposons que A soit" was a very common mistake, and that the correct form is :
    "supposons que A est".
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