Dr. Jacobson, an emeritus professor of mathematics at Yale, was regarded as one of the world's leading researchers in abstract algebra. He was best known for his research in basic algebraic structures called rings. Among his leading contributions in this area were the Jacobson Density Theorem, a fundamental theorem in the discipline, and the Jacobson radical, a mathematical structure that plays an important role in analyzing rings.

''If you understand the Jacobson radical, then you have an entree into understanding the ring more completely,'' said Dr. Michael Rosen, Dr. Jacobson's son-in-law, who is a professor of mathematics at Brown University. ''It's part of the algebraic language one learns.''

Dr. Jacobson was also known for his political efforts on behalf of Jewish mathematicians from the Soviet Union, many of whom he helped bring to the United States. In the early 1970's, when he was a vice president of the International Mathematical Union, he became embroiled in a battle with the organization's other vice president, L. S. Pontrjagin of the Soviet Union, over that nation's refusal to allow many of its mathematicians, mostly Jews, to address or even attend meetings of the International Congress of Mathematicians. The matter was never officially resolved.

Dr. Jacobson was a member of the National Academy of Sciences and the American Academy of Arts and Sciences, and last year won the American Mathematical Society's highest honor, the Leroy P. Steele Prize for Lifetime Achievement.

He was also known to mathematics students around the world as the author of more than a dozen standard textbooks, and many of the 30 or so graduate students he advised have become prominent mathematicians.

Dr. George Seligman, a student of his who later became his colleague at Yale, recalls visiting him in the final stages of thesis work, frustrated by the many details still to nail down. When he complained, Dr. Jacobson replied, ''Well, you're young.''

''I interpreted that as saying that if it's worth doing, then it's worth knocking yourself out to do,'' Dr. Seligman said.

Born in Warsaw in 1910, Dr. Jacobson came to the United States with his family in 1918. He was reared in Mississippi and Alabama, and graduated from the University of Alabama in 1930. He received a doctorate in mathematics from Princeton in 1934, advised by Dr. Joseph Wedderburn, who proved some of the most basic results in the theory of algebras. Dr. Jacobson later extended and generalized Dr. Wedderburn's work.

Dr. Jacobson taught mathematics at Bryn Mawr College, the University of North Carolina and Johns Hopkins University before being hired at Yale in 1947. In 1963, he was named Henry Ford II professor of mathematics. He retired from Yale in 1981.

Dr. Jacobson's wife of 54 years, Florence Dorfman Jacobson, also a mathematician, died in 1996. He is survived by a son, Michael, of Norwalk, Conn.; a daughter, Pauline Jacobson of Providence, R.I.; a granddaughter, and three great-grandchildren.

By SARA ROBINSON, December 9, 1999 © The New York Times Company