Taxation and the sustainability of collusion: ad valorem versus specific taxes

Assuming constant marginal cost, it is shown that a switch from specific to ad valorem taxation that results in the same collusive price has no effect on the critical discount factor required to sustain collusion. This result is shown to hold for Cournot oligopoly when collusion is sustained with Nash-reversion strategies or optimal-punishment strategies. In a Cournot duopoly model with linear demand and quadratic costs, it is shown that the critical discount factor is lower with an ad valorem tax than with a specific tax that results in the same collusive price. However, in contrast to Colombo and Labrecciosa (J Public Econ 97:196–205, 2013) it is shown that the revenue is always higher with an ad valorem tax than with a specific tax.     

The optimality of optimal punishments in Cournot supergames

The result of Colombo and Labrecciosa [Colombo, Luca and Labrecciosa, Paola (2006). ‘The suboptimality of optimal punishments in Cournot supergames’, Economics Letters 90, pp. 116–121.] that optimal punishments are inferior to Nash-reversion trigger strategies with decreasing marginal costs is shown to be due to the output when a firm deviates from the punishment path becoming negative.