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