SAMPLED-DATA SIMULATION OF PERIODICALLY SWITCHED NONLINEAR CIRCUITS - Computer Methods for Analysis of Mixed-Mode Switching Circuits - page 147

Neglecting the interpolation error, the behavior of the second-order
Volterra circuits in the simulation window is depicted by
where is a constant vector specifying the nodes to which the input of
the second-order Volterra circuit is connected. Eq.(7.24) is solved using
the sampled-data simulation and the principle of superposition