Three results on the regularity of the curves that are invariant by an exact symplectic twist map . Invariant subspaces for operators in a general II[1]-factor . Weakly commensurable arithmetic groups and isospectral locally symmetric spaces . The C[1] generic diffeomorphism has trivial centralizer . Finitude homotopique et isotopique des structures de contact tendues . Deux caractérisations de la mesure d'équilibre d'un endomorphisme de P[k](C) (Erratum)