Leopold B. Felsen, 81, a physicist and engineer who was a highly respected authority on electromagnetic waves, died Sept. 24 at a hospital in Boston. He had been ill with muscular dystrophy for 30 years.

Dr. Felsen was perhaps best known in the world of science as co-author of "Radiation and the Scattering Waves" (1973), a seminal work of numerical algorithms that helped solve long-held questions, among them the behavior of radiation waves as they encounter various obstacles.

The findings in the book, co-written with physicist Nathan Marcuvitz, helped scientists make technological advances in numerous applications, including remote sensing, microwave engineering and wireless communication.

"He was always interested in using mathematics to solve problems," said his son, Michael Felsen. "And physics was just one way of doing it."

Dr. Felsen was educated and spent most of his career at what is now the Polytechnic University of New York, where he was dean of the faculty of engineering in the mid-1970s and a teaching professor until his retirement in 1994.

Despite his poor health, he continued to work, including teaching in Boston University's Department of Aerospace and Mechanical Engineering, writing books and technical articles, giving lectures and attending conferences in the United States and abroad.

Shortly before his death, he edited a colleague's paper in the hospital and was collaborating on a book, his son said.

Dr. Felsen was born in 1924 in Munich, where his Jewish parents operated a textile business. He was 16 years old when he was sent to the United States to live with relatives. Other family members, including a sister, died in the Holocaust.

In New York, his proficiency in mathematics led him to skip high school and go directly to City College of New York, which he attended at night before entering the Army during World War II. In the military, he trained as an electronics technician, calibrating firearms.

After the war, he concentrated on academics, receiving bachelor's, master's and doctoral degrees from what was then the Polytechnic Institute of Brooklyn.

Dr. Felsen was an author of more than 300 scientific papers and several books, a life fellow of the Institute of Electrical and Electronic Engineers, an elected member of the National Academy of Engineering and a fellow of the Acoustical Society of America and the Optical Society of America.

His interest in writing extended beyond technical papers. He wrote poetry, slogans and reflections that he shared with colleagues, friends and family.

One of his favorites dealt with humanity: "We are all born human, but we have to work to become humane."

His wife, Sima Felsen, died in 1975.

In addition to his son, of Boston, survivors include a daughter, Judith Felsen of Bartlett, N.H.; and three grandchildren.

Leopold Felsen "was always interested in using mathematics to solve problems," said son Michael Felsen.