The price of imperfect competition for a spanning network

a r t i c l e i n f o a b s t r a c t JEL classification: C72 C79 D40 D43 D44 Keywords: Algorithmic mechanism design Worst case scenario equilibrium analysis Frugality Minimum cost spanning tree problem Price of imperfect competition A buyer procures a network to span a given set of nodes; each seller bids to supply certain edges, then the buyer purchases a minimal cost spanning tree. An efficient tree is constructed in any equilibrium of the Bertrand game. We evaluate the price of imperfect competition (PIC), namely the ratio of the total price that could be charged to the buyer in some equilibrium, to the true minimal cost. If each seller can only bid for a single edge and costs satisfy the triangle inequality, we show that the PIC is at most 2 for an odd number of nodes, and at most 2 n−1 n−2 for an even number n of nodes. Surprisingly, this worst case ratio does not improve when the cost pattern is ultrametric (a much more demanding substitutability requirement), although the overhead is much lower on average under typical probabilistic assumptions. But the PIC increases swiftly when sellers can only provide a subset of all edges.