After facing a string of vacancies due to retirements, the Mathematics Department is rebuilding its ranks this year with the additions of one tenured professor and six assistant professors.

Bruce Kleiner, a former University of Michigan professor who specializes in geometry, accepted a senior appointment in the department last spring and is teaching a year-long graduate course on differential geometry. The assistant professors were hired into the department’s Gibbs program on three-year terms.

Andrew Casson, chair of the math department, said the department is still looking to fill four senior positions by the end of the year. He said he hopes to restore the department’s offerings in algebra, which have weakened over the years.

“There was a great tradition of algebra at Yale, and all the algebrists have retired now roughly speaking,” Casson said. “Algebra number theory and algebraic geometry are the particular areas in which we’re looking.”

The department, which has 16 senior faculty slots, currently has 12 of those slots filled. In December 2003, three math professors retired, and algebra expert Serge Lang retired from the department last year. Yale College Dean Peter Salovey said the vacancies in the department are the result of “math cohorts” that have all reached retirement age at Yale within the last decade.

“The age distribution in a department is not random, and departments often go through periods of great hiring when a field is expanding,” Salovey said. “The result is that losses in departments due to retirement and deaths are not random either.”

Despite the shortage of professors, Casson said the department has largely been able to cover all of its bases because many of its faculty members are qualified to teach a wide variety of courses. Still, the department is looking for a professor to teach an introductory number theory course, one that traditionally is taught by visiting professors, Casson said.

The department currently offers relatively few introductory courses — a three-course calculus series that can be entered at different levels and an introductory fractal geometry course. But Casson said the shortage of professors has made it difficult to lay out new courses.

“I think most of the courses currently offered are feasible,” Casson said, adding that math professor Michael Frame will teach a freshman seminar entitled “Geometry of Nature” this semester. “I don’t think we have plans to make more introductory courses.”

Although the department is still short of a few courses this semester, Casson said professors like Gregory Margulis — who is one of few people who have won both the Wolf Prize and the Fields Medal for his work in mathematics — continue to draw students into the department.

“We do have an extremely distinguished faculty,” Casson said.

Kevin Wortman, a new Gibbs assistant professor, said he is excited to begin teaching during the spring semester. A graduate of the University of Chicago, Wortman did his post-doctorate work at Cornell University but decided to leave when he received an offer to come to Yale last March.

“The research interests of this department were better aligned with my research interests than those at Cornell, so I decided to come here,” Wortman said.

Wortman will teach two undergraduate courses, on linear algebra and Galios Theory, for math majors next semester.

Salovey said he is optimistic about the department’s prospects of hiring some younger professors this year.

“There has been some very good success in bringing in some young senior math professors in recent years,” Salovey said. “We’re really looking for the world’s best mathematicians regardless of seniority, but we’re always delighted to find world-class faculty who can be at Yale a good long time.”