sentences:  Returning to Odessa, he became a merchant, but in 1878 he again took up his scientific studies, and for the next four years busied himself at Heidelberg, especially with mathematics and physics.In 1880, with Lazarus Fuchs as thesis advisor, he earned his doctorate with the dissertation "Lineare homogene Cofunktionen".In 1883 he established himself as privat-docent in mathematics at the University of Heidelberg, becoming assistant professor in 1887.