Perhaps you should try some spelling variations:
"operational systems" » "operating systems", "relational systems", "operation system"
"functional systems" » "national systems", "educational systems"
"operation systems" » "operations systems", "operation system", "cooperation systems"
"function systems" » "reaction systems", "detection 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: (((("operation systems") OR ("operational systems"))) OR ((("functional systems") OR ("function systems")))).
• Removing quotes may allow a broader search: (((( operation systems ) or ( operational systems ))) or ((( functional systems ) or ( function systems )))).
• Adding a wildcard symbol may retrieve word variants: (((("operation systems") or ("operational systems"))) or ((("functional systems") or ("function systems"))))*.