With the emergence of decentralized finance, new trading mechanisms called
Automated Market Makers have appeared. The most popular Automated Market Makers
are Constant Function Market Makers. They have been studied both theoretically
and empirically. In particular, the concept of impermanent loss has emerged and
explains part of the profit and loss of liquidity providers in Constant
Function Market Makers. In this paper, we propose another mechanism in which
price discovery does not solely rely on liquidity takers but also on an
external exchange rate or price oracle. We also propose to compare the
different mechanisms from the point of view of liquidity providers by using a
mean / variance analysis of their profit and loss compared to that of agents
holding assets outside of Automated Market Makers. In particular, inspired by
Markowitz' modern portfolio theory, we manage to obtain an efficient frontier
for the performance of liquidity providers in the idealized case of a perfect
oracle. Beyond that idealized case, we show that even when the oracle is lagged
and in the presence of adverse selection by liquidity takers and systematic
arbitrageurs, optimized oracle-based mechanisms perform better than popular
Constant Function Market Makers.