The nearest airport is DUS (Düsseldorf). After arrival, take the SkyTrain to the “Düsseldorf Flughafen” railway station. From there, you have the option of several trains (RE or S1) to get to Essen’s central station, “Essen Hbf”. Here is a timetable for Friday morning.
Arrive to Essen’s central station, “Essen Hbf”. You can purchase tickets at the Deutsche Bahn website.
From Essen Hbf:
After arriving to the central station, take the subway (U-Bahn) to the station “Berliner Platz”. It will be just two stops on any of the lines U11 (direction: Buerer Str.), U17 (direction: Karlsplatz), or U18 (direction: Berliner Platz).
From Berliner Platz, it is a 500 m walk to the mathematics department, which is located at Thea-Leymann-Straße 9. Follow the directions on Google Maps.