= Bare coupling
{title2=$\lambda_0$}
A <bare coupling> is an interaction coefficient in a regulated action. It is related to a <renormalized coupling> through counterterms and any required powers of the <renormalization scale>. For quartic scalar theory in $4-\epsilon$ dimensions, its <mass dimension> is $\epsilon$, and $\lambda_0=\mu^\epsilon[\lambda_R+a\lambda_R^2/\epsilon+\cdots]$. At fixed regulator, changing $\mu$ while leaving the theory unchanged keeps $\lambda_0$ fixed; the corresponding change of $\lambda_R$ defines its <renormalization-group beta function>.
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