Before revision:  Hence, market efficiency at least requires the absence of weak arbitrage opportunities. I show that no weak arbitrage is necessary but not sufficient to establish a situation where asset prices always "fully reflect" a specific set of information beyond the price history.
After revision:  Hence, market efficiency at least requires the absence of weak arbitrage , but this is not sufficient to establish a situation where asset prices always "fully reflect" a specific set of information beyond the price history.