Perhaps you should try some spelling variations:
common mathematical » non mathematical
theory student » theory students, theory studying, theory study
theory studien » theory studies, theory studie, theorien studien

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: common mathematical ((theory student) OR (theory studien)) texts.
• Adding a wildcard symbol may retrieve word variants: common mathematical ((theory student) or (theory studien)) texts*.