 Complete the following phrase:  Popescu was appointed as a Lecturer at the University of Bucharest in 1968 where he taught graduate students until 1972. Starting in 1964 he also held a research appointment at the Institute of Mathematics of the Romanian Academy. The institute was closed in 1976 by order of Nicolae Ceaușescu for reasons related to his daughter Zoia Ceaușescu, who had been hired at the institute two years before, but was reopened in 1990, after the 