Collusion in differentiated duopolies revisited

In a letter previously published in this journal, Rothschild (1992) [Rothschild, R., 1992. On the sustainability of collusion in differentiated duopolies. Economics Letters 40, 33–37] claims to obtain results in contrast with those obtained by Deneckere (1983) [Deneckere, R., 1983. Duopoly supergames with product differentiation. Economics Letters 11, 37–42]. We show however that, taking correctly into account non-negativity constraints on output, Rothschildʹs formulation yields the same conclusions as Deneckereʹs.