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