A counterexample to the quantum Hedetniemi conjecture
Godsil, Roberson, \v{S}\'amal and Severini conjectured that the quantum chromatic number of the categorical product of two graphs equals the minimum of the quantum chromatic numbers of the factors. We disprove this conjecture: we construct explicit finite graphs $G,H$ with \[ \chi(G\times H) \leq 1538<1539 = \min(\chi_...