Motivation. That elegant fiction the competitive equilibrium seems still
to dominate the frontiers of theoretical microeconomics. We may think of
it in a general way as a state of affairs wherein economic agents,
responding "rationally" to annoWlced prices, make choices which are
consistent and feasible. The prices may also be described as "taken" for
one reason or another the agents who respond to them consider them as
given. The existence of such a state, its optimality, its robustness
against free bargaining among agents when there are many of them, its
Wliqueness, its stability when price displacements evoke specified
adjustments--all these issues have been studied, and continue to be
studied in a variety of settings. Slowly the equilibrium investigated
begins to incorporate public goods, externalities of certain kinds,
differences in agents' information, and infinitely many time periods.
The appeal of such results need not be belabored: the equilibrium
studied may sustain an optimal resource allocation, and when it does it
sus- tains it in a manner that appears to be informationally efficient
and to accord well with individual incentives. Therefore it is important
to extend the circumstances under which an equilibrium exists, under
which it sustains opti- mality, and under which it survives
displacements as well as free bargaining among agents.