  In 1999 Robert Fernholz observed an inconsistency between the normative
assumption of existence of an equivalent martingale measure (EMM) and the
empirical reality of diversity in equity markets. We explore a method of
imposing diversity on market models by a type of antitrust regulation that is
compatible with EMMs. The regulatory procedure breaks up companies that become
too large, while holding the total number of companies constant by imposing a
simultaneous merge of other companies. The regulatory events are assumed to
have no impact on portfolio values. As an example, regulation is imposed on a
market model in which diversity is maintained via a log-pole in the drift of
the largest company. The result is the removal of arbitrage opportunities from
this market while maintaining the market's diversity.
