Dr. Hugh Everett III, 51, a mathematical physicist who was the founder and chairman of the DBS Corp., an information science and data management company in Arlington, died July 19 at Fairfax Hospital after a heart attack. He lived in McLean.

Dr. Everett was an authority in the fields of quantum mechanics and operations research and the author of works on the theory of games and the effect of fallout in nuclear warfare.

He joined the staff of the Institute for Defense Analyses in 1956. He rose to the post of director of the mathematics and physical sciences division of the weapons systems evaluation group.

In 1964, he left the government to become director and then president of the Lambda Corp., a company specializing in defense analysis work and the applications of computer sciences to government work. He resigned from Lambda when he founded DBS in 1973.

Dr. Everett also was a vice president of American Management Systems Inc. and the founder and owner of the Key Travel Agency, both in Arlington.

Dr. Everett was a native of Washington and a magna cum laude graduate of Catholic University, where he earned a degree in chemical engineering. He received a master's degree in physics at Princeton University in 1955 and two years later got his doctorate in mathematical physics at Princeton.

Survivors include his wife, Nancy Gore Everett, and two children, Elizabeth A. and Mark O., all of McLean, and his stepmother, Sara T. Everett of Berryville, Va.