Since June 2001 he has served as Governing Mayor ( '' '' ) of Berlin .
Since June 2001 he has served as Governing Mayor , `` Regierender Bürgermeister '' , of Berlin .
He studied mathematics from 1975 to 1979 in Chemnitz . In 1980 , he went to Rostov on Don to study and earned a PhD there in 1984 .