sentences:  In 1948 he was awarded a doctorate as a Doctor of Philosophy with cum laude for his thesis on the rate of circulation of money and its bearing on the value of money and monetary equilibrium.In the same year he was appointed professor of economics at the Vrije Universiteit.Representing the Anti-Revolutionary Party, Zijlstra successively served as Minister of Economic Affairs in the Drees II, Drees III and Beel II cabinets, and as Minister of Finance in the Beel II and De Quay cabinets between 2 September 1952 and 24 July 1963.