Perhaps you should try some spelling variations:
"cooperating systems" » "cooperation systems", "operating system", "cooperative system"
"operational systems" » "relational systems", "operation system", "conversational systems"
"operation systems" » "operations systems", "operation system", "cooperation systems"
"cooperative systems" » "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 ("cooperative systems"))))) OR ((("operational systems") OR ("operating systems")))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"operation systems\") or (((\"cooperating systems\") OR (\"cooperative systems\"))))) or (((\"operational systems\") or (\"operating systems\"))))".
• Removing quotes may allow a broader search: (((( operation systems ) or ((( cooperating systems ) OR ( cooperative systems ))))) or ((( operational systems ) or ( operating systems )))).
• Adding a wildcard symbol may retrieve word variants: (((("operation systems") or ((("cooperating systems") OR ("cooperative systems"))))) or ((("operational systems") or ("operating systems"))))*.