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