  Applicability of three popular functions (Vogel-Fulcher-Tammann, double
activation law and frustration-limited domains model) for the description of
the temperature dependence of alpha-relaxation time tau_alpha is considered for
three typical glass-formers (propylene carbonate, ethanol and picoline). Two
first functions have three free parameters. It was found that while they are in
satisfactory agreement with the experimental data of tau_alpha(T), they fail in
describing the transition from an Arrhenius-like to a non-Arrhenius behaviour.
This transition is seen in the derivative analysis of tau_alpha(T). We argue
that Vogel-Fulcher-Tammann and double activation functions should be applied
and compared only at T < T_A, where T_A is the temperature of transition from
an Arrhenius-like to a non-Arrhenius behaviour. It was shown that the
four-parametric frustration-limited domains model with imbedded transition from
Arrhenius to non-Arrhenius behaviour at T = T* also has no advantage in the
derivative analysis, since T* is systematically different from TA in the cases
considered.
