  A detailed theoretical analysis of the Gaussian fluctuations of the order
parameter in layered superconductors is performed within the Ginzburg-Landau
(GL) theory. The available results for the Gaussian fluctuations are
systematized and a number of novel formulae for the fluctuation magnetization,
nonlinear magnetic susceptibility, heat capacity and high-frequency
conductivity in layered superconductors are derived. We propose several new
prescriptions: how to determine the life-time constant of fluctuation Cooper
pairs tau_0, the in-plane coherence length xi_ab(0), the energy cutoff
parameter e_cut=c hbar^2 / 2m_ab xi^2_ab(0), and the Ginzburg number e_Gi. It
is demonstrated, for example, how the spectroscopy of the life-time can be used
to verify the existence of depairing mechanisms in layered cuprates. The
ultraviolet regularization of the GL free energy is carried out by means of the
well-known from the field theory zeta-function method. We further show that the
archetype of the latter method has its origin in the century of enlightenment
and the novel result is that the fluctuation part of the thermodynamic
variables of the layered superconductors can be expressed in terms of the Euler
Gamma-function and its derivatives. Universal scaling curves for the magnetic
field dependence of the paraconductivity sigma_ab(T_c,B), fluctuation
magnetization M(T_c,B) and the heat capacity C(T_c,B) are found for the
quasi-2D superconductors at T_c and further related to the form-factor of the
Cooper pairs.
