= Solution
A momentum-shell <renormalization group> transformation consists of three steps. First split $\phi=\phi_-+\phi_+$ into slow modes with $|k|<\Lambda/\zeta$ and fast modes with $\Lambda/\zeta<|k|<\Lambda$, and integrate out $\phi_+$ to obtain a <Wilsonian effective action> for $\phi_-$. Next rescale momenta by $k'=\zeta k$, equivalently coordinates by $x'=x/\zeta$, to restore the cutoff to $\Lambda$. Finally rescale the field to restore the chosen normalization of the gradient term. Repeating these operations produces a <renormalization-group flow> of every permitted mass and coupling.
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