Ermolaev B.I. (Phys.Techn.Inst., St. Petersburg)

Comment on the use of fixed/frozen couplings in QCD

As is well-known the QCD coupling alpha_s is running. The only rigorously known expression for $\alpha_s$ comes from RGE. Nevertheless the approximation of fixed coupling is quite often used in QCD in order to simplify calculations. In this case the scale of alpha_s is conventionally fixed a posteriori from phenomenological considerations. We argue that there are different frozen couplings for calculations in the Double- and Single- Logarithmic Approximations. We present estimates for their values.