Equilibrium in closed constant-function market maker economies

Abstract

We study equilibria in a closed, fee-free constant-function market maker (CFMM) economy with two assets and two agents. An interior state is a unilateral no-trade equilibrium exactly when the CFMM marginal price equals both agents' marginal rates of substitution. For an interior initial state, individually rational unilateral equilibria are Pareto optimal relative to the fixed CFMM invariant. A weak representative agent is obtained at each fixed equilibrium by weighted sup-convolution, whereas, under some regularity assumptions, a state-independent strong representative agent exists exactly when agents share the same homothetic preference. Every interior feasible state is reachable through finitely many valid trades, but an individually rational interior target need not be improvement reachable. Alternating utility-maximizing trades converge to a Pareto optimal unilateral equilibrium.