Grünbaum–Nash-Williams conjecture
In graph theory, the Grünbaum–Nash-Williams conjecture states that every 4-vertex-connected toroidal graph has a Hamiltonian cycle. It is a generalization of Tutte's theorem on Hamiltonian cycles, according to which every 4-vertex-connected planar graph has a Hamiltonian cycle.
Source: Wikipedia — Grünbaum–Nash-Williams conjecture (CC BY-SA 4.0)