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