Coase's 'theorem' says that under certain conditions (low 'transaction costs', 'full information'?), bargaining can be expected to produce Pareto efficient outcomes. Of course, Coase did not prove a theorem as such. My question, then, is whether Coase's argument has been formalised into a theorem.
Before I pose the question, let me briefly comment on what do not seem to be adequate formalisations:
I remember hearing that Coase's argument is 'just' an application of the First Welfare Theorem (FTW). The idea is that, if there is a market for all goods (e.g. for pollution rights), then we can just apply the FTW to show that the outcome must be efficient. This strikes me as completely wrong. The FWT assumes that outcomes are generated through competitive markets (e.g. there must be a market for 'pollution rights' whose price everyone takes as given). If you read Coase, however, you'll see that this is absolutely not what he had in mind. Instead, Coase imagined a process of bargaining (not price taking!); e.g. the polluter might be bribed to cut back on their pollution.
One might alternately try to formalise Coase within the context of a bargaining model, e.g. Rubinstein's alternating offers model. Such models can yield efficient outcomes (e.g. this happens under alternating offers). However, I believe that Coase's insight is not supposed to depend on the exact details of the bargaining process; so one might worry that this approach fails to correctly capture the key to his argument.