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