Jun-ichi Igusa, a retired Johns Hopkins university professor of mathematics who researched number theory and algebraic geometry, died of a stroke Nov. 24 at the Holly Hill Nursing Home. The Hunt Valley resident was 89.

"He was a giant in his field," said Bernard Shiffman, chair of the Hopkins mathematics department. "He was meticulous in everything he did. Even when he taught elementary calculus, he was thorough and prepared his classes perfectly. He was warm to people and interested in helping his students."

Colleagues said that several mathematical concepts, the Igusa curves and the Igusa zeta function, were named in his honor. His work later assisted software engineers to encrypt passwords and other pieces of sensitive information.

He was born in a village in the Gunma prefecture in Japan. Family members said he grew up in the countryside. His father cultivated silk worms and made a living selling their cocoons to weavers.

"As a student, my father built radios, telescopes and scientific devices from wood or pieces of wire and screws he found around the house. He might buy the tubes," said his son, Takeru Igusa, who lives in Stoneleigh. "Years later, he wrote that his early experiences in building them helped lay the foundation for his mathematical thinking."

He was a 1945 graduate of the University of Tokyo and earned a doctorate from Kyoto University in 1953. He became a professor of mathematics at the University of Tsukuba. He came to Harvard University and taught there briefly before Wei-Liang Chow, then the chair of the Hopkins mathematics department, recruited him in 1955.

"If it was mathematics, he could speak the language," said Takeru Igusa. "Everyday English was more difficult for him, but he mastered it. He often wrote out what he was going to say so he could speak it perfectly."

His family recalled that Dr. Igusa was a careful scientist who left little to chance.

"Everything he did was carefully planned out," said another son, Kiyoshi Igusa of Newton, Mass. "Even the classes he taught. He always wrote his lecture out in advance, even the jokes."

Dr. Shiffman said that Dr. Igusa's roots in a rural community made him uncomfortable with more polished persons who lived in cities.

"He told me that he compensated for this by being very thorough and working things through many times over," Dr. Shiffman said.

"I was honored to have been his student," said Don McQuillan, emeritus professor of mathematics at the National University of Ireland in Dublin. "A distinguished and world-famous mathematician, he was also a kind, patient and generous thesis director."

After his 1993 retirement from Hopkins, Dr. Igusa wrote books in his field. He joined 29 other mathematicians to help prove Fermat's Last Theorem. This work became the subject of an episode of a Nova broadcast on PBS.

He was given the title of professor emeritus.

Dr. Igusa was named an inaugural Fellow of the American Mathematical Society in 2012. He also served as editor-in-chief of the American Journal of Mathematics.

He was the founding director of the Japan-U.S. Mathematics Institute at Hopkins. The institute fosters collaboration between Hopkins math department faculty members and mathematicians from Japan and other countries.

Dr. Igusa addressed the International Congress of Mathematicians in 1962. His work, "Theta Functions" was published in 1972.

"I will always be grateful to Professor Igusa for introducing me to his beautiful mathematics and for inspiring me to solve some of his fascinating problems," said Diane Meuser, a Boston University associate professor of mathematics.

Dr. Igusa was given one of Japan's highest honors, the Order of the Sacred Treasure, for his role in cultivating scientific exchange between Japan and the U.S in 2005.

He enjoyed fishing at Loch Raven Reservoir and along the Gunpowder Falls. He studied the vineyards of France and made numerous trips there.

"He and my mother drove through the countryside. They ate good food and drank good wine," said Takeru Igusa.

Funeral services are private. His Hopkins colleagues will hold a memorial at a later date.

In addition to his sons, survivors include his wife of 65 years, Yoshie Yamamoto; another son, Mitsuru Igusa of Los Gatos, Calif.; and seven grandchildren.