Momentum-shell decomposition of a scalar field
= Momentum-shell decomposition of a scalar field
{title2=$\Phi=\phi_<+\phi_>$}
Split <Fourier transform> modes into a low-momentum ball and its disjoint shell. The quadratic scalar action has no cross term between these supports. Integrating the shell at fixed low field defines the lower-cutoff <Wilsonian effective action>.