The price after the theoretical price of futures index moves up by one transaction cost is called "the upper bound of no arbitrage interval", and the price after the theoretical price of futures index moves down by one transaction cost is called "the lower bound of no arbitrage interval". Only when the actual futures index is higher than the upper limit can forward arbitrage make a profit. On the contrary, only when the actual futures index is lower than the lower limit can reverse arbitrage make a profit.