We express connected Fermionic Green's functions in terms of completely explicit tree formulas. In contrast with the ordinary formulation in terms of Feynman graphs these formulas allow a completely transparent proof of convergence of the perturbative series. Indeed they are compatible with the mathematical implementation in Grassmann integrals of the Pauli principle. The first formula we give, which is the most symmetric, is compatible with Gram's inequality. The second one, the "rooted formula", respects even better the antisymmetric structure of determinants, and allows the direct comparison of rows and columns in the fermionic determinant. To illustrate the power of these formulas, we provide a ``three lines proof'' that the radius of convergence of the Gross-Neveu field theory with cutoff is independent of the number of colors. An other potential area for applications would be the study of correlated Fermions systems and superconductivity.