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