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

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: (((("operation systems") OR ("cooperating systems"))) OR ("operational systems")).
• Removing quotes may allow a broader search: (((( operation systems ) or ( cooperating systems ))) or ( operational systems )).
• Adding a wildcard symbol may retrieve word variants: (((("operation systems") or ("cooperating systems"))) or ("operational systems"))*.