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