{c}
{title2=$u_N=\sum_{k\in K_N}a_k(t)\phi_k$}

A <Fourier truncation> retains a finite set of boundary-compatible <Fourier modes> and projects the PDE onto their span by the Galerkin method. Nonlinear products can generate unretained harmonics, so their coefficients are discarded by projection rather than declared zero pointwise. Agreement with a perturbation expansion through cubic order requires retention of every quadratically forced mode that feeds back on the critical mode at cubic order.

// Destination: dynamical-systems.bigb


 Fourier truncation (source code)