“…The rear ring then passes through the leading ring, with this process of 'leapfrogging' then repeating again and again [23]. We note that interactions of such vortex rings -primarily with filament cores -have been considered in Hicks [20] and more recently in Borisov, Kilin, and Mamaev [13], in Caplan, Talley, Carretero-González, and Kevrekidis [14], in Cheng, Lou, and Lim [15] and in Aiki [4].…”