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