Perhaps you should try some spelling variations:
del litterature » de litterature, des litteratures, et litterature
studies data » studies ds, studies d, studies dn
test files » text files, test file
studies da » studies dna, studies das, studies dan

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: test files ((studies da) OR (studies data)) filosofia del litterature.
• Adding a wildcard symbol may retrieve word variants: test files ((studies da) or (studies data)) filosofia del litterature*.