# Soit A une partie de N

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#### john_riemann_soong

##### Senior Member
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

##### Member
"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!

#### astermarc

##### New Member
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.

#### Fred_C

##### 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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