California Atty. Gen. Bill Lockyer said his investigators are looking into the antitrust implications and the potential effect on gasoline prices of the recently proposed merger between BP Amoco and Los Angeles-based Atlantic Richfield Co. Lockyer said state law gives him the authority to block a merger if he concludes that it would result in a monopoly. More likely, the inquiry will result in the state's attempting to force the new company to sell some of its gas stations to ensure competition. Lockyer noted that Arco long has sold gasoline at prices below those charged by other major retailers. "Now, with Arco being absorbed by one of the higher-priced retailers, we could lose the impact of the Arco floor," Lockyer said. He also intends to seek a legally binding agreement with the new company that it maintain Arco's tradition of charitable giving. Arco's shares fell $1.38 in New York Stock Exchange trading, finishing at $76.31.