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We obtain the corrections in $1/n$ and in $1/\ln n$ ($n$ is the principalquantum number of the bound state) of the decay constants of scalar andpseudoscalar currents in two and four dimensions in the large $N_c$. We obtainthem from the operator product expansion provided a model for the large $n$mass spectrum is given. In the two-dimensional case the spectrum is known andthe corrections obtained in this paper are model independent. We confirm theseresults by confronting them with the numerical solution of the 't Hooft model.We also consider a model at finite $N_c$ and obtain the associated decayconstants that are consistent with perturbation theory. This example shows thatthat the inclusion of perturbative corrections, or finite $N_c$ effects, to theOPE does not constrain the slope of the Regge trajectories, which remain a freeparameter for each different channel.

1/N_c and 1/n preasymptotic corrections to Current-Current correlators