The L − ΔL plane for a single perturber: a bar (left panel) and a four-armed spiral wave (right panel), from the test-particle simulations presented in Minchev & Famaey (2010). The bar and spiral pattern speeds and strengths are indicated by Ω_b,s and ϵ_b,s, respectively. The dotted-red vertical lines indicate the CR of each perturber and the solid-blue ones – the ILR and OLR. The color bar shows the normalized particle density. Such a shape of the distribution (indicated by the green line), with an increase in ΔL inside and a decrease outside the CR is characteristic for a single perturber, but not when perturbers interact as discussed in the text.