Strategists at Citigroup believe that the sell-off in U.S. stocks last week has left major indexes vulnerable to further declines. Strategists such as Chris Montagu said in a note to clients on Monday that the large number of long positions squaring in the S & P 500, combined with an increase in short positions in the Nasdaq 100, suggest a shift in risk appetite towards a more direct bearish bias. They said hedge fund de-risking operations in the S & P 500 have left total exposures at half their mid-July peak.