isochrone
1977 / Gravitational fields whose brachistochrones and isochrones are identical curves / Ramesh Chander / American Journal of Physics 45 (1977) p. 848 [doi]
1979 / Comment on ''Gravitational fields whose brachistochrones and isochrones are identical curves'' / John E. Prussing / American Journal of Physics 47 (1979) p. 117 [doi]