On the LPENS intranet you can find instructions for useful computer system resources.
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To obtain Mathematica got to the page Service de Prestations Informatiques and look for the procedure under Licenses home-use. Note : this can be done only within ENS.
You will be asked to fill a form with your ENS informations and it will automatically forward the request to the "Correspondant logiciel de l'ENS" for approval. Wait for the e-mail containing the license (it can take also a week), create a Wolfram ID, acces Wolfram User Portal, register the license you just received and download the installer.
You can get a CNRS zoom account, which allows you to host meetings that are longer than 40 minutes. You can follow the information here: http://www.dr8.cnrs.fr/CNRS-Hebdo/Actualites/12238/Suite.aspx