Simple test model for the periodicity generated in the outside disc, i.e. at r > a. In the top panel we show the temporal evolution of the torque, whereas in the bottom panel we show the Fourier transform. In each panel, the blue curve is obtained by placing three point masses at distance 1.6a, 2.1a, 3.5a from a circular binary, and the one red is the torque found in the iso10 simulation.