Abstract
The crossover temperature between the mean-field and the full-critical regions for the thermal fluctuations around the superconducting transition is calculated for the first time in single-layered and bilayered (with two different distances and tunneling couplings between adjacent layers) superconductors at zero applied magnetic field. These results show, in particular, that the width of the full-critical region is somewhat smaller in layered superconductors than in equivalent bulk materials. These results are then discussed in connection with the superconducting transition of the high-Tc copper oxide superconductors.