A $2/3$ Bound for Vizing's Conjecture

Vizing's conjecture, dating back to 1963, asserts that \[ γ(G\mathbin{\square}H) \geq γ(G)γ(H) \] for all finite graphs $G$ and $H$, where $γ$ denotes the domination number and $\square$ denotes the Cartesian product. In 2000, Clark and Suen proved the universal bound \[ γ(G\mathbin{\square}H) \geq