 This machine has two states, indicated by circles. It begins in the leftmost state. The \(\bC\) in the left circle means that the machine cooperates on the first move. The arrow leading from the left to the right circle indicates that machine defects (enters the \(\bD\)) after it has cooperated (been in the \(\bC\) state) and its opponent has defected (the arrow is labeled by \(d\)). Linster has conducted simulations of evolutionary PD's among the strategies that can be represented by two-state Moore machines. It turns out that these are exactly the deterministic versions of the \(\bS\) strategies of Nowak and Sigmund. Since the strategies are deterministic, we must distinguish between the versions that cooperate on the first round and those that defect on the first round. Among the first round cooperators, \(\bS(1,1,1,1), \bS(1,1,1,0), \bS(1,1,0,1)\) and \(\bS(1,1,0,0)\) all represent the strategy \(\bCu\) of unconditional cooperation. Similarly, four of the first-round defectors all represent \(\bDu\). Each of the other \(\bS(p_1, p_2, p_3, p_4)\), where \(p_1,p_2,p_3,p_4\) are either zero or one represent unique strategies, and each comes in two varieties according to whether it cooperates or defects in round one. By deleting the six duplicates from these thirty-two deterministic versions of Nowak and Sigmund's strategies, we obtain the twenty-six “two-state” strategies considered by Linster.