Hörmander applied for professorship at Stockholm University, but temporarily left for the United States while the request was examined. He spent quarters from winter to fall in respective order at the University of Chicago, the University of Kansas, the University of Minnesota, and finally at the Courant Institute of Mathematical Sciences in New York City. These locations offered "much to learn" in partial differential equations, with the exception of Chicago of which he however notes the Antoni Zygmund seminar held by Elias Stein and Guido Weiss to have strengthened his familiarity with harmonic analysis.