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Ermolaev B.I. (Phys.Techn.Inst., St. Petersburg)
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Comment on the use of fixed/frozen couplings in QCD
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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.