Perhaps you should try some spelling variations:
"operational systems" » "operating systems", "relational systems", "operation system"
"cooperation system" » "cooperative system", "operation system", "cooperative systems"
"operations systems" » "operations system", "operating 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 system") OR ("operational systems"))) OR ("operations systems")).
• Removing quotes may allow a broader search: (((( cooperation system ) or ( operational systems ))) or ( operations systems )).
• Adding a wildcard symbol may retrieve word variants: (((("cooperation system") or ("operational systems"))) or ("operations systems"))*.