Byrne, P.C.: 'Stability of 2-channel nonlinear systems with symmetric linear parts', Electronics Letters, 1976, 12, (4), p. 95-96, DOI: 10.1049/el:19760074
IET Digital Library, https://digital-library.theiet.org/;jsessionid=v9ka48mp0lbx.x-iet-live-01content/journals/10.1049/el_19760074