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