This person showed that any  Walrasian equilibrium is Pareto-efficient, thus proving the fundamental theorem of welfare economics, with Kenneth Arrow.
This thinker introduced a coefficient of resource utilization to evaluate the deadweight loss associated with Pareto non-  optimality. Expanding on a result by Sonnenschein, this person proved with Mantel that Walrasian identity and  continuity characterize the aggregate excess demand function in a result that nevertheless shows that individual  rationality assumptions do not imply uniqueness or stability of general competitive equilibria. With an American, this  person proved that demand independence, perfect competition, and convex preferences guarantee the existence of at least  one set of prices such that aggregate supply and demand are (*) equal for all commodities. This person showed that any  Walrasian equilibrium is Pareto-efficient, thus proving the fundamental theorem of welfare economics, with Kenneth Arrow. For  10 points, name this winner of the 1983 Nobel for his reformulation of general equilibrium theory, a French-born economist.
This thinker introduced a coefficient of resource utilization to evaluate the deadweight loss associated with Pareto non-  optimality.
Gerard Debreu