Hi everybody, I am trying to get addtionnal data on "infinitesimal numbers" dx. I am not sure about the terminology, I have heard it a long ... long time ago during a lecture (my memory may be wrong, so may be I was sleepind and it was during a dream? ). I think (memory) that the definition may be something like "whatever e>o, 0<|dx|<e" where dx is the "object" satisfying such definition (i.e. the "infinitesimal number" definition may be the "smallest" open set in a topology?). Does anyone can provide some information about this? Thanks a lot in advance. Seratend.