Perhaps you should try some spelling variations:
"conversational systems" » "conversion systems"
"operations system" » "operations systems", "operation system", "operational system"
"operating systems" » "operation systems"

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: (((("operating systems") OR ("operations system"))) OR ("conversational systems")).
• Removing quotes may allow a broader search: (((( operating systems ) or ( operations system ))) or ( conversational systems )).
• Adding a wildcard symbol may retrieve word variants: (((("operating systems") or ("operations system"))) or ("conversational systems"))*.