Perhaps you should try some spelling variations:
cordel literature » border literature, core literature, model literature
level literature » legal literature, levels literatur, novel literature
duales studium » das studium
test duales » test doubles, test rules, test oracles

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 duales studium _ filosofia ((level literature) OR (cordel literature)).
• Adding a wildcard symbol may retrieve word variants: test duales studium _ filosofia ((level literature) or (cordel literature))*.