“…: universal, direct simulation, O(t 2 ), [65] : universal, 2-tag simulation, O(t 4 log 2 t), [4,27,91] : universal, bi-tag simulation, O(t 6 ), [69] : semi-weakly universal, direct simulation, O(t 2 ), [105] : semi-weakly universal, cyclic-tag simulation, O(t 4 log 2 t), [111] : weakly universal, Rule 110 simulation, O(t 4 log 2 t), [68] universal non-universal Despite the existence of these efficient O(t 2 ) simulators, it still remained the case that the smallest universal machines were exponentially slow. However, we have recently shown that the smallest machines are in fact efficient simulators of Turing machines, by showing that 2-tag systems are efficient [108].…”