Perhaps you should try some spelling variations:
del literatur » model literatur, teil literatur, igel literatur
tests safety » test safety, guests safety, stents safety
studien am » studien aus, studien auf, studien _
studien a » studien _, studien 3, studien 4

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

• The words AND, OR and NOT may confuse the search; try adding quotes: "tests safety ((studien a) OR (studien am)) filosofia del literatur".
• Adding a wildcard symbol may retrieve word variants: tests safety ((studien a) OR (studien am)) filosofia del literatur*.