Title : Big denominators and analytic normal forms Speaker: Laurent Stolovitch (Universit\'e de Nice - Sophia Antipolis) Abstract: We study the regular action of an analytic pseudo-group of transformations on the space of germs of various analytic objects of local analysis and local differential geometry (for instance, vector fields vanishing at one points, singularities of functions,...). We fix a homogeneous object $F$ and we are interested in an analytic normal form for the whole affine space $F_0 + h.o.t.$. We prove that if the cohomological operator defined by $F_0$ has the big denominators property and if a formal normal form is well chosen then this formal normal form holds in analytic category. We also define big denominators in systems of nonlinear PDEs and prove a theorem on local analytic solvability of systems of nonlinear PDEs with big denominators. We illustrate our theorems by explanation of known results and by new results in the problems of local classification of singularities of vector fields, non-isolated singularities of functions.