= Solution
A <momentum-shell renormalization group> step has three parts. First split the <Fourier transform> of the field into slow modes $\phi_<$ with $|k|<\Lambda/\zeta$ and fast modes $\phi_>$ with $\Lambda/\zeta<|k|<\Lambda$, then perform the <functional integral> over $\phi_>$. Second rescale momenta by $k'=\zeta k$, equivalently coordinates by $x'=x/\zeta$, to restore the cutoff from $\Lambda/\zeta$ to $\Lambda$. Third rescale the field so that the coefficient of $(\nabla\phi)^2/2$ again has its chosen normalization. The effective free energy contains every operator allowed by the symmetries, with transformed coefficients. Repeating the step composes these coefficient maps and produces a <renormalization-group flow>.
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