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