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