  In the framework of the 1/N-expansion we show that the Callan-Symanzik beta-
function associated with the four-point coupling g is non-analytic at its zero,
i.e. at the fixed-point value g^* of g. This behavior can be interpreted by
renormalization group arguments, and written in terms of scaling correction
exponents. We obtain accurate determinations of g^* in 3-d and 2-d by
exploiting two alternative approaches: the (4-d)-expansion and the
high-temperature expansion. These results are compared with the available
estimates by other approaches, such as the fixed-dimension perturbative
expansion, Monte Carlo simulations, etc... We also present results for the
n-point renormalized coupling constants that parameterize the behavior of the
effective potential in the high- and low- temperature phases.
