Garey and Johnson showed shortly afterwards in 1974 that the directed Hamiltonian cycle problem remains NP-complete for planar graphs and the undirected Hamiltonian cycle problem remains NP-complete for cubic graphs.
Garey and Johnson showed shortly afterwards in 1974 that the directed Hamiltonian cycle problem remains NP-complete in some important special cases, such as cubic graphs and planar graphs. They also showed that the undirected Hamiltonian cycle problem remains NP-complete in cubic graphs.