DOUBLE PRECISION FUNCTION XQS(QLAM,LOOPFL,NF) - Calculation of $4\pi/\alpha_{\overline{\rm MS}}(Q^\prime/\Lambda_{\overline{\rm MS}}^{(n_f)})$

In this routine the inverse running coupling ${\rm XQS}=4\pi/\alpha_{\overline{\rm MS}}$ is calculated as a function of ${\rm QLAM}=Q^\prime/\Lambda_{\overline{\rm MS}}^{(n_f)}$. For this end, the explicit formula Eq. (9.5a) in Ref. [1] for the running coupling is used, which is accurate to 3-loop.

The integer variable LOOPFL contrôls the loop-order up to which we use Eq. (9.5a) in Ref. [1].