Perhaps you should try some spelling variations:
"cooperation systems" » "cooperation system", "cooperative systems", "operation systems"
"cooperating systems" » "operating systems", "cooperative systems", "operating system"
"operational systems" » "operating systems", "relational systems", "operation system"

Try widening your search to All Fields.

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: (((("cooperation systems") OR ("cooperating systems"))) OR ("operational systems")).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"cooperation systems\") OR (\"cooperating systems\"))) or (\"operational systems\"))".
• Removing quotes may allow a broader search: (((( cooperation systems ) OR ( cooperating systems ))) or ( operational systems )).
• Adding a wildcard symbol may retrieve word variants: (((("cooperation systems") OR ("cooperating systems"))) or ("operational systems"))*.